Leela and the Genestalk - S7-E22
Trivia: When Fry, Bender, and Leela come into Mom's control room, the background sounds of the Enterprise (TOS) can be heard. Also in this episode, while Fry and Bender climb up the side of the castle, Adam West and Burt Ward make a cameo appearance as genetic experiments. They popped out of the window as the celebrities did in the original Batman series.
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]