Put Your Head on My Shoulder - S2-E10
Other mistake: When Fry is talking to Leela (After being stitched onto Amy's body), he asks for some privacy from Amy, who puts her fingers in her ears and begins singing a song in Chinese. At one point, when it shows a close-up of Fry's head, you can barely see Amy's head, and the finger disappears and reappears near her nose, then suddenly reappears back in her ear.
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]