Roswell That Ends Well - S4-E1
Factual error: There were no nuclear tests anywhere near the Roswell Air Force base in New Mexico.
Roswell That Ends Well - S4-E1
Continuity mistake: Leela sets fire to the Professor's tie in the white goods store - later in the Malt Shop, his tie is completely intact.
Answer: You are thinking about episode 20 of series 2 - "Anthology of Interest I". She technically never kills anyone, but in the episode she ask a "What if machine" (one of the Professor's inventions) what would happen if she was more impulsive. The machine then shows the more impulsive Leela killing off the Professor to get the money from his will and thereafter killing Hermes, Bender, Scruffy, Cubert, Nibler, Amy and Zoidberg to cover it up. Finally she does something really impulsive with Fry.
Andreas[DK]