Fry: It's just like the story of the grasshopper and the octopus. All year long, the grasshopper kept burying acorns for the winter, while the octopus mooched off his girlfriend and watched TV. But then the winter came, and the grasshopper died, and the octopus ate all his acorns. And also he got a racecar. Is any of this getting through to you?
I, Roommate - S1-E3
Bender: Fry, of all the friends I've had... You're the first.
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]