The Simpsons is known for its sometimes edgy storylines. The show does not shy away from themes like murder, especially with its Treehouse of Horror Halloween episodes. Groundskeeper Willie seems especially suspect — he's a bit rough around the edges whenever he appears as part of The Simpsons. With his shady background, a case could be made that Willie is a serial killer.
There are several instances throughout The Simpsons that make Willie look especially guilty. First, in Season 14, Episode 8, “The Dad Who Knew Too Little," it is revealed that Willie has a remarkable resemblance to the Aberdeen Strangler, a former Scottish serial killer. But the circumstantial evidence goes even beyond his physical appearance.
Second, Willie is confronted in Season 24, Episode 16, “Dark Knight Court," by Marge and Lisa. He has evidence of the crime on his kilt, revealing him as the culprit behind the eggs in the children’s instruments. He exclaims that “the world is better off with four fewer Welshmen” before asking what Marge actually meant.
Finally, the most damning evidence for the likelihood that Willie is a serial killer is that he has killed before. In Season 22, Episode 12, “Homer the Father," it's revealed that Willie once killed a boy on the school’s playground. He claims it was an accident, but the fact that the boy was impaled with a rake makes this claim at least seem dubious. If this is not bad enough, Willie has also expressed the wish to kill Mr. Burns in Season 6, Episode 25, “Who Shot Mr. Burns – Part One," for getting Willie fired from the school due to budget cuts. He also expressed a desire to murder everybody else in town while running for mayor in Season 5, Episode 7, “Bart’s Inner Child."
Technically, a killer has to commit three murders to be classified as a serial killer. Since only one murder has been confirmed, Willie wouldn't be labeled as such. However, it could be that The Simpsons just hasn't revealed his other murders yet. Willie's remark about the four Welshmen alone would put Willie over the benchmark. While it's obvious that he has committed murder, Willie's serial killer status remains unknown — at least for now.