Continuity mistake: When Bender is a "female" and is putting stuff in his locker, in one angle you can see a yellow item next to his pink handbag. Then in the wider angle, this item is gray.
Continuity mistake: Right at the end, Bender is smoking a cigar and says something like "Emotions should be hated." In the very next shot when he has stood up, the cigar has vanished from his hand and there isn't anywhere he would have put it down.
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]