Leela and the Genestalk - S7-E22
Continuity mistake: When Leela's parents call Fry on her "wrist thingie", we see it has clamps on the back that go like "up down up down" but in the middle of the sequence they change to "up down down up". (00:12:50)
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]