Where No Fan Has Gone Before - S4-E12
Continuity mistake: When the Planet Express Ship first lands on the planet, it crashes with noticeable damage. When they try to sneakily escape while the screen play is going on (before they get the parts from the Star Trek ship), the damage is repaired.
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]