Continuity mistake: When Fry comes into the kitchen while Leela is making toast, he walks over to the cup rack to take a cup. When it cuts, the rack has changed position on the wall. (00:06:20)
Continuity mistake: During the Mother's Day commercial on TV, as Destructor hands over a card to Mom, the antenna on the TV suddenly changes color without the shot cutting. (00:01:00)
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]