Spanish Fry - S5-E12
Trivia: When Lrr and Nd-Nd are dining at the park, trying to get in "the mood", Lrr gets very agitated at his wife. He jumps up and screams to her "One of these days, Nd-Nd, bang. Zoom. Straight to the third moon of Omicron Persei 8." This is a spin-off from the 50s TV show The Honeymooners, in which one of the characters says almost the exact same thing to his wife, Alice.
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]