In-show deaths

Even though The Simpsons primarily maintains the status quo, throughout the run of the series, several characters, both recurring and one-time, have died. Additionally, multiple characters have been jokingly killed off during the Treehouse of Horror episodes.

Deaths during the series

Before Season 1/Unknown

Hubert Simpson, died in the The Tracey Ullman Show shorts episode "The Funeral".
Clancy Bouvier, in "Jazzy and the Pussycats" Marge says that he died shortly after she married Homer.
Snowball I, died after being hit by a car, being driven by Clovis Quimby. The death is first mentioned in "Simpsons Roasting on an Open Fire" with Clovis Quimby being mentioned as the killer in "Treehouse of Horror III".
Great-Grampa Simpson, seen in Heaven by Bart in "Bart Gets Hit by a Car".
Johnny, killed by the Vietnamese in a flashback in "I Love Lisa".
Milton Haas, died from a hernia while carrying the crate of stolen Nazi paintings out of the German castle.
Sheldon Skinner, Arnie Gumble, Iggy Wiggum, Etch Westgrin and Griff McDonald, killed during the 1979 Veterans Day float disaster.
C.W. McAllister, died on the Murderhorn mountain in 1928, presumably from hypothermia. Abraham Simpson has taken a bite out of his arm. His body was only found in "King of the Hill".
Waylon Smithers, Sr., died of radiation poisoning. When he was younger, Homer Simpson discovered his body, presumably a while after his death as it had quite substaintually decayed. Homer had flashbacks of the event when he was hypnotised in "The Blunder Years". Mr. Burns later showed the Simpson family, as well as Chief Wiggum and Waylon Smither Jr., the footage of his death. Showing that he had died saving Springfield.
Mr. Teeny I, II and III, Krusty remarks that he hasn't been this sad since Mr Teeny III died in "Fraudcast News".
Pa, died at some point before "How Munched Is That Birdie in the Window?"
Viktor's wife, died at some point before "The Falcon and the D'ohman"

Season 1

No deaths.

Season 2

Beatrice Simmons, died of a burst ventricle "Old Money", although Abraham Simpson says that it was actually a "broken heart", after he was unable to attend her birthday party .

Season 3

Snuffy, Lisa's pet hamster. She recites a poem about his death in "Stark Raving Dad".
Stinky and Wrinkles, Bart kills the school fish with a yo-yo, whilst trying the 'Plucking the pickle' trick in "Bart the Lover".

Season 4

Aunt Gladys Bouvier, died in "Selma's Choice".

Season 5

Great Uncle Boris, left the Simpsons family his house when he died in "Homer Loves Flanders".

Season 6

Bleeding Gums Murphy, died in hospital from illness in "'Round Springfield".
SuperDude, the school gerbil, crushed by his water bottle in "Who Shot Mr. Burns? (Part One)".
Dr. Marvin Monroe, It is unknown whether Marvin Monroe is actually dead. In "Who Shot Mr. Burns? (Part Two)", the hospital Burns attends is called The Marvin Monroe Memorial Hospital, and since that, his gravestone has been seen. Most prominently, in "The Simpsons 138th Episode Spectacular" one of the trivia questions asked was "Which two popular characters have died in the past year?" With the answer being, "If you said Bleeding Gums Murphy and Dr. Marvin Monroe you were wrong, they were never popular," indicating he is dead. However, Marvin Monroe re-appeared ten years later in the episode "Diatribe of a Mad Housewife", saying that he has been "very sick". If his death had taken place, it is estimated to have happened some time during Season Six.

Season 7

Great-Aunt Hortense, died sometime before "Bart the Fink" and left the Simpsons $100 each.
Asa Phelps, died in "Raging Abe Simpson and His Grumbling Grandson in "The Curse of the Flying Hellfish"", leaving Abraham Simpson and Charles Montgomery Burns as the last two surviving members of their army unit, and the last two in line to inherit the stolen Nazi paintings.
Mr. Estes, a publications advisor, died and left Lisa Simpson in charge of the school year book, sometime before the episode "Summer of 4 Ft. 2".

Season 8

Frank Grimes tombstone.

James Bont, a James Bond parody, was shot to death by Hank Scorpio's guards in "You Only Move Twice".
Convict, was about to be executed in Springfield State Prison's electric chair when Hurricane Barbara lifted the roof off the prison and pulled him into the air, ostensibly to safety. He subsequently landed on some power lines and was electrocuted, in "Hurricane Neddy".
Frank Ormand, the man who gave Marge Simpson the idea to begin a pretzel business. Died in a car crash in "The Twisted World of Marge Simpson".
The executor of Frank Ormand's will, died in a car crash along with Frank Ormand, in "The Twisted World of Marge Simpson".
Shary Bobbins, died after being sucked into a jet engine in "Simpsoncalifragilisticexpiala(Annoyed Grunt)cious".
Rex Banner, presumably died when he was catapulted out of town in "Homer vs. the Eighteenth Amendment".
Frank Grimes, died of electrocution after touching some cables marked "Extremely High Voltage" without safety gloves, whilst impersonating Homer Simpson in "Homer's Enemy". He was alluded to later on in the series in the episode "Natural Born Kissers" when Homer found a program from his funeral in a suit jacket pocket, as well as in "The Great Louse Detective" when Frank Grimes's son tries to murder Homer to avenge his father's death.
Poochie, died on the way back to his home planet during an episode of the Itchy & Scratchy Show in "The Itchy & Scratchy & Poochie Show". He was killed off due to immense unpopularity. However, he still appears as a background character from time to time, as being a cartoon character he can't actually die.
The Fish in Disco Stu's shoes, died presumably because of using up all of the oxygen in their water, or from constant trauma, considering they were in Aquariums in Disco Stu's shoes. Stu explained that he "couldn't get them out of there".

Season 9

Captain Tenille, presumeably died when Homer Simpson torpedoed him out of a submarine in "Simpson Tide".

Season 10

Mr. Pinchy, Homer Simpson's pet lobster, was accidentally cooked to death when the bath water he was in was too hot. An extremely upset Homer ate Mr. Pinchy later at dinner in "Lisa Gets an "A"".
Red Barclay, died in "Maximum Homerdrive" after a beef-eating contest. Dr. Hibbert, a partner of the steakhouse, says he died of beef poisoning, although it was "probably from some other restaurant".
Mother bird, Bart Simpson accidentally shoots the bird with Nelson Muntz's BB gun. The gun had a crooked sight, which meant that despite the fact that Bart was aiming away from the bird, he still killed it. Bart had to look after her eggs, in "Bart the Mother".

Season 11

Maude Flanders, died in "Alone Again, Natura-Diddily" when she was knocked off a racetrack grandstand by t-shirts shot from cannons. She was killed off due to problems with her voice actress Maggie Roswell. Maude's death is considered to be the most show-changing death, and affected Ned Flanders in several episodes after the incident.
Cornelius Chapman, the only man in Springfield older than Mr. Burns (at 108 years old). He died onstage from a heart attack after a kiss on the cheek from Britney Spears. Fittingly enough, he was receiving an award for Springfield's Oldest Man in "The Mansion Family".
The Laramie Cigarette Team died when the helicopter exploded at the hands of a Tomacco addicted sheep (who was the only survivor.)

Season 12

Man having operation, died when a tennis ball landed in his open chest in "Tennis the Menace", after Dr. Hibbert decided to operate on him outside.

Season 13

Professor Huntington, was dying of cancer, in "Homer the Moe" he kills himself by walking in to a lake. He later appears as a ghost.

Season 14

Congressman Horace Wilcox, died of a stroke in "Mr. Spritz Goes to Washington".
Andy, died because failed with Moe's drink-a-gallon-of-gin challenge "'Scuse Me While I Miss the Sky".

Season 15

Lawyer, died, when Homer drove over him in My Mother The Carjacker.
Snowball II Run over by Dr. Hibbert's Mercedes-Benz in "I, (Annoyed Grunt)-bot".
Snowball III, a ginger cat that was the replacement of Snowball II. It died when it drowned in the Simpsons fish tank in "I, (Annoyed Grunt)-bot".
Coltrane, a.k.a. Snowball IV, a white cat and the replacement of Snowball III, it died when it jumped out of the window to escape Lisa's music in "I, (Annoyed Grunt)-bot". It was replaced by a cat that looked exactly like Snowball II, which Lisa then names it "to save money on a new food bowl".
Presumably everyone within the Dayton Arms Hotel, when it was obliterated on screen (all the way to the basement) in "Catch 'Em If You Can".
Elmo, hung himself in "Bart-Mangled Banner", after being wrongly imprisoned at the Ronald Reagan Reeducation Center.

Season 16

Frankie, the health inspector, died when eating one of Moe Szyslak's pickled eggs in "Mommie Beerest".
Los Lobos, along with several other unnamed celebrities, died after falling out of a blimp.

Season 17

Man In Grave, a man who was buried alive by Gravedigger Billy in "The Girl Who Slept Too Little".
Fred Kanneke, Chief Wiggum's brother-in-law who was given euthanasia.

Season 18

The Calabresis (Dante Calabresi, Sr. and Dante Calabresi, Jr.), the rival gang of Fat Tony's, died of food poisoning.
The Calabresis' Goons, died in a car explosion.
Amber Simpson, Homer's Vegas wife died of an overdose whilst in front of a "Do not stand up on the rollercoaster" sign.
Max, an ice cream truck driver. Died whilst giving Homer change for a $100.
Diver, drowned, when Nelson stole his equipment.

The Simpsons Movie

Green Day, died when the boat they were playing on flipped over. The Springfield residents later pray for them in church.
An unnamed man, crushed by the dome as it was being installed over Springfield.

Season 19

People Reviewing Sundance Films, died from loss of breath in "Any Given Sundance".
Mona Simpson, dies of natural causes in "Mona Leaves-a".

Season 20

Howard, Sr., died sometime before "Wedding for Disaster".

Season 21

Annie, eaten by Santa's Little Helper in "American History X-cellent".
Bluella, died in "The Squirt and the Whale" after she was washed up on the shore of Springfield.

Season 26

Hyman Krustofsky, died in "Clown in the Dumps".
Squawky, died in "The Musk Who Fell to Earth", when sucked into the engine of Elon Musk's flying vehicle.

Season 27

Nibbles, died in "Lisa the Veterinarian", extremely depressed and ill.

Treehouse of Horror deaths

Note: Treehouse of Horror deaths are kept separate due to the nature of Treehouse of Horror episodes. Treehouse of Horror episodes aren't continuous with the rest of the series, giving the writers the possibility of killing off important characters without having to create a replacement or write them out of the series (e.g. in Treehouse of Horror XV, Homer obliterates Springfield, but the series continues on.)

"Treehouse of Horror"

Lenore, referred to as "Lost Lenore" in "The Raven" segment.

"Treehouse of Horror II"

No deaths.

"Treehouse of Horror III"

Lenny Leonard, eaten by King Homer.
Waylon Smithers, Jr., eaten by King Homer.
Shirley Temple, eaten by King Homer.
Journalist, crushed by King Homer.
Clancy Bouvier, eaten by King Homer.
Numerous zombies (Ned Flanders, Albert Einstein, William Shakespeare, Edna Krabappel, and George Washington to name a few), killed mainly by Homer when zombies attack Springfield.

"Treehouse of Horror IV"

Hans Moleman , his car explodes after being knocked off the road by Otto Mann.
Groundskeeper Willie's mule, Willie comments that he had to put seventeen bullets in him.
Ned Flanders, killed by a gremlin through decapitation.
Charles Montgomery Burns (Vampire), died of a stake through heart by Homer after first staking his crotch.

"Treehouse of Horror V"

Groundskeeper Willie, stabbed in the back with an axe in each of the three mini-episodes, all while trying to help the main character of the episode. In the third death he replies that he's "bad at this."
Patty Bouvier, revealed to have died in an alternate universe. The cause of her death, however, is not mentioned.
Selma Bouvier, revealed to have died in an alternate universe. The cause of her death, however, is not mentioned.
Various students of Springfield Elementary School including Jimbo Jones, Üter, Wendell, Milhouse, Bart, and Lisa, killed by the school staff. It is possible that all of the children were killed and eaten, with the exception of Ralph who was never shown to be sent to detention.
- Note that the people killed in the third segment didn't die because it was in Bart's nightmare.

"Treehouse of Horror VI"

Groundskeeper Willie, burnt to death after Homer Simpson turned the heating up to high. However, he showed up just fine at the end of the episode, getting off a bus. He attempted to scare Bart, Lisa, and Maggie, but was last seen chasing the bus he got off of because he had forgotten he left his gun on the seat.
Martin Prince, strangled to death by Willie.
Kent Brockman, killed by a billboard version of himself when it came to life, although he is seen alive later on in the same segment.
Otto Mann and a school bus of students, killed by a billboard of the Devil come to life.
Captain of the High School Basketball team, shot by Chief Wiggum who thought he was a monster.
Homer Simpson, although he didn't technically die, he was trapped in a different dimension so in the Simpson's world he would be listed as dead.

"Treehouse of Horror VII"

Bill Clinton, ejected into space, completely naked.
Bob Dole, ejected into space, completely naked.

"Treehouse of Horror VIII"

FOX Censor, stabbed to death.
All of Springfield except Homer, Marge, Bart, Lisa and Maggie Simpson, killed by a nuclear neutron bomb. Others including, Lenny, Carl, Sideshow Mel, Mr. Burns, Dr. Hibbert, Ned Flanders, Chief Wiggum, Apu Nahasapeemapetilon, Waylon Smithers, Jr., Principal Skinner, Moe Szyslak, Groundskeeper Willie, and the Sea Captain were all mutated and later killed by the Simpsons.
Johnny and Edgar Winter, run over by Homer, who mistook them for mutants.
Agnes Skinner, Miss Hoover, and Luann Van Houten, burned alive.

"Treehouse of Horror IX"

Homer Simpson, hit by Marge Simpson's car.
Lisa Simpson, crashed into a wall on her bike.
Bart Simpson, accident on his skateboard.
Snake Jailbird, electrocuted in an electric chair.
Apu Nahasapeemapetilon, drowned in a Squishee machine by Homer Simpson, who was being controlled by Snake Jailbird's hair.
Moe Szyslak, Homer Simpson (who was being controlled by Snake Jailbird's hair) pulled his heart out with a corkscrew.
Jerry Springer, suffocated by Kang's tentacle, he was also possibly killed from a venomous bite of the alien/human hybrid Maggie Simpson.
Audience of the Jerry Springer show, zapped by laser gun.
Poochie, hit by a car. However Poochie grunted after the impact.

"Treehouse of Horror X"

Lisa Simpson, disintegrated by an alien Maggie from "Treehouse of Horror IX".
Ned Flanders, hit by a car. However, he reappeared as a zombie werewolf, so now he is undead.
Comic Book Guy as The Collector, killed by Lucite.
Homer Simpson, blown up after escaping the spaceship going into the sun, died because of no oxygen in space.
Bart Simpson, blown up after escaping the spaceship going into the sun, died because of no oxygen in space.
Celebrities on doomed spaceship, died when their spaceship flew into the sun.
Most people on earth, killed while all of the world's machines go haywire.

"Treehouse of Horror XI"

Homer Simpson, killed by an angry mob.
Bart Simpson, killed by an angry mob.
Marge Simpson, killed by an angry mob.
Abraham Simpson, killed by an angry mob.
Homer Simpson, choked to death on a piece of broccoli, twice.
Agnes Skinner, dropped to her death by Ghost Homer when he tries to do a good deed.
Simpson children, Bart and Lisa see their siblings skeletons.
Goldilocks, mauled by Daddy Bear, Mummy Bear and Baby Bear.
Suzanne the witch, burnt to death.
Lenny Leonard, killed by dolphins.
Captain McCallister, killed by dolphins.
Squeaky-voiced teen, killed by dolphins.
Kent Brockman, killed by dolphins.
Staff Worker, killed by dolphins.
Abraham Simpson, eaten by a dolphin, though a deleted scene shows him bursting out of the dolphin and joining in Springfield's battle against the dolphins.
Groundskeeper Willie, killed by dolphins.
Krusty the Clown, killed by dolphins.
Many other citizens of Springfield, killed by dolphins.

"Treehouse of Horror XII"

Waylon Smithers, Jr., electrocuted.
Lenny Leonard, crushed by a helicopter.
Carl Carlson, crushed by a helicopter.
Bart Simpson, drowned himself in his cereal.
Ralph Wiggum, killed by Lord Montymort.
Lord Montymort, killed by Bart Simpson.
Moe Szyslak, drowned in a pickled egg jar.

"Treehouse of Horror XIII"

Homer clone, electrocuted.
Ned Flanders, decapitated with a chainsaw by Homer clone.
Homer clones, killed by Homer.
Gil, killed by Homer clones, although he was still moving and speaking.
Homer clones, fall off a cliff.
Homer Simpson, falls off a cliff.
Zombies, shot.
Future Homer Simpson, shot by Moe.
Professor Frink, eaten after being turned into a turkey.

"Treehouse of Horror XIV"

Abraham Simpson, burned to death(though he just sits there complaining that he's still cold).
Homer Simpson, shot.
Grim Reaper, murdered by Homer with a bowling ball.
Homeless Guy, killed by Homer the Grim Reaper.
Jasper, killed by Homer the Grim Reaper.
Thirteen men (including Kirk Van Houten, Reverend Lovejoy and Hans Moleman) and three women (including the Queen), killed during a ball game (all by Homer, except the Queen).
Patty Bouvier, killed by Homer (although he was meant to kill Marge; Patty had Marge's hair on the top)
Ned Flanders, killed by Professor John Frink Sr after he rips out his heart.
Seymour Skinner, killed by Professor John Frink Sr. after he rips out his spine.
Many scientists, killed by Professor John Frink Sr.
Professor John Frink Sr., died form a calculated kick to the groin from Professor John Frink Jr.
Martin Prince, died, or was seriously injured while being attacked by the angry mob.

"Treehouse of Horror XV"

The Simpson family (except for Bart), cooked by Kang and Kodos.
Dr. Hibbert, falls out of a window.
Hans Moleman, dropped down a sewer and devoured by crocodiles.
Homer Simpson, shot by Ned thrice.
All of Springfield, killed by Homer when he pressed the "Core Destruct" button as he fell down dead.
Prostitute, killed by Clancy Wiggum.
Ned, Todd and Rod Flanders, their heads hang in Comic Book Guy's shop.
Comic Book Guy, killed by Clancy Wiggum.
Selma Bouvier, killed by Clancy Wiggum.
- It should be noted that the last four deaths technically did not occur, as they were all part of Ralph Wiggum's dream.

"Treehouse of Horror XVI"

David, sliced in half by Bart.
All adult males (with the exception of Homer), shot by Mr. Burns (note: Moe was not killed by Mr. Burns, he was impaled by a weather vane and Clancy Wiggum was also impaled; also, Professor Frink was eaten by Homer, not killed by Mr. Burns).
Disco Stu, arrow through the head. (a la Steve Martin)
Mayor Quimby, eaten by dogs.
Martin Prince, sprayed by Groundskeeper Willie.
Abraham Simpson, It is unclear as to whether he is killed or not, but it is likely he would be listed as dead after Dr. Hibbert (who had became a vampire) flew off into the night with him. Abraham said "That's it, suck out the poison...Wait a minute...Ahhhhh!" Implying that he had either been turned into a vampire (Which would technically make him undead) or had been killed due to blood loss.

"Treehouse of Horror XVII"

Moe Szyslak, killed by an Iron maiden (torture device)
Snowball II (Snowball V), eaten by Homer Simpson.
Squeaky Voiced Teen, eaten by Homer.
Üter, eaten by Homer.
German People at Oktoberfest, eaten by Homer.
Fat Tony, eaten by Homer.
Comic Book Guy, eaten by Homer.
People in Umpire School, eaten by Homer.
People from "The Facts of Life Reunion Tour", eaten by Homer.
Guys in Hawaiian shirt store, eaten by Homer.
People on a bus, eaten by Homer.
Ned Flanders, had his head bitten off by Homer.
Doctor Phil McGraw, eaten by Homer.
Homeless People, eaten by Homer.
17th Century People, shot by the Golem of Prague in a flashback.
Audience members at Krusty's stand up show, shot by the Golem of Prague.
Principal Skinner, sliced in half by the Golem of Prague. (However, he still moved before being put in a bag, but he is listed by Chief Wiggum as dead, so he is probably killed by the Golem.)
Lenny Leonard, shot by Homer Simpson, who thought he was an alien.
Disco Stu, shot by Homer, who thought he was an alien.
Captain McAllister, killed by Homer.
Clancy Wiggum, sucked dry by Kang and Kodos's spaceship.
Most Springfieldians, killed in Kang and Kodos' invasion.
Lou, Killed when he walked into the lake to get Chief Wiggum's sweater.

"Treehouse of Horror XVIII"

Three soldiers, killed by Kodos.
Many Rigellians (including Kang and Kodos), killed by Homer
Kent Brockman, assassinated by Marge.
Five men and Captain MacAllister, accidentally shot by Homer.
Grampa, accidentally killed by Homer as he tries to kill Marge.
Chief Wiggum, killed by Marge.
Principal Skinner, killed by Marge and Homer.
Groundskeeper Willie, killed by his killer tractor.
Dr. Hibbert, run down by a van.
Homer, killed when he is filtered through his own hammock.

"Treehouse of Horror XIX"

Homer, killed by voting booth.
Grandpa Simpson, possibly died after passing out in the Simpson's front yard.
People On Plane, coughed out by Plane Robot onto runway.
Various People Throughout Springfield, probably killed during robot battle.
Krusty the Clown, shredded by a woodchipper.
George Clooney, Prince, Neil Armstrong, and other celebrities, Killed by Homer (George Clooney fell in quicksand, prince was strangled with his guitar by Homer, and Neil was hit and became unconscious from a flying golf club).
Several People at Party, Shot at by deceased stars that came down from heaven.
Chief Wiggum, shot by the deceased Edward G. Robinson.
Homer, shot by the deceased Krusty the Clown.
Principal Skinner, killed by Grand Pumpkin.
Grand Pumpkin, killed by Tom Turkey.
Several Students at the Party, eaten by Tom Turkey.

"Treehouse of Horror XX"

Homer Simpson, head gets chopped off and it falls in the punch bowl at the party.
Miss Hoover, killed by Bart.
Principal Skinner, killed by Bart.
Bart Simpson, accidentally stabbed by Lisa.
Everyone except Homer, Marge, Bart, Lisa, & Maggie, turned into munchers and eventually got shot.
Homer Simpson(in "There's No Business Like Moe Business" play), falls into boiler room under Moe's Tavern and his body goes through pipes that have beer in them.

"Treehouse of Horror XXI"

Professor John Frink, killed himself by a DVR remote.
Chief Wiggum, squashed by a giant Knight piece.
Milhouse Van Houten, drowned and later, hung by the Hangman
Rod and Todd Flanders, squashed in a tumbling block game.
Cards, killed by Bart
Bart and Milhouse, hung
Seagull, poisoned
Shark, Squid and other sea life, killed by fuel from the Rent-A-Dream.
Roger, harpooned
Albatross crew, various - anchor, flare gun, blunt weapon
Pelican - Marlin
Ned Flanders - cooked
Big Bird - killed by Count von Count
Homer - when a bat, could not fly
Count Dracula and Edmund (possibly) - Homer's blood.

"Treehouse of Horror XXII"

Mr. Burns - killed by Ned Flanders.
Sideshow Bob - killed by Ned Flanders.
Patty and Selma Bouvier - killed by Ned Flanders.
Homer Simpson - killed by God.
Gary Chalmers - fell off a cliff in a robotic suit.

"Treehouse of Horror XXIII"

The entire population of Earth: Killed during the 13th baktoon.
Sideshow Mel: Only one killed by the black hole (Had his skeleton sucked out)
Chief Wiggum: Killed by the devil.

"Treehouse of Horror XXIV"

The Fat In The Hat - killed by Maggie.
Bore-Ax - killed by The Fat In The Hat.
Krusty Korral - killed by The Fat In The Hat.
Mr. Burnsum - killed by The Freaks.

References

SNPP 'Characters on the Show' deaths SNPP
SNPP 'THOH' deaths SNPP