He does not do a very good job, so Bart decides to scare an alternative lunch out of his friends by making up a story about a cannibal cafeteria worker named Dark Stanley, who killed all the students in the cafeteria and put them in his kids' head soup and was eventually hanged for his crimes.
Since he doesn't do a great job, Bart fabricates a story about Dark Stanley, a cannibal cafeteria worker who murdered the students and cooked them into his kids' soup, resulting in his death by hanging.