Dark Stanley

Stanley DeGroot, better known as Dark Stanley, was a chef at Springfield Elementary School.

History
In a story told by Bart, Dark Stanley never graduated from college and all the kids made fun of him. Eventually, he snapped and killed the kids with a cleaver. He then made "Kids Head Soup" and stated that it needed more girl.

A psychologist also suggested to Stacey Swanson that Bart could be replacing her son, who was killed by Dark Stanley.

Behind the Laughter
Although originally implied to be just a story told by Bart, Dark Stanley is later implied to be real as another psychologist asked Stacey Swanson if Bart was replacing her son, who was killed by Dark Stanley.