Continuity mistake: The amount of canisters Leela has opened at the end is constantly changing.
Continuity mistake: When Leela revives Fry at the end, she is right next to him. When Fry regains consciousness, Leela is at least a half foot away from him without enough time to move.
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]