Roswell That Ends Well - S4-E1
Factual error: There were no nuclear tests anywhere near the Roswell Air Force base in New Mexico.
Anthology of Interest 2 - S4-E3
Factual error: When human-Bender is dancing in the disco with the two girls there is a fully lit disco ball which manages to cast no light beams on the walls and floor, as they normally do.
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]