Where No Fan Has Gone Before - S4-E12
Trivia: As Sulu is telling the story, the vessel firing the rocket with the "sacred texts" of 79 episodes, six movies, and blooper reel, with minor differences, very closely resembles the Eagle vehicles in the British Sci-Fi series Space: 1999.
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]