Continuity mistake: When Fry and Leela dig out Santa there are cracks in the ice around him which disappear when they lift him up.
Continuity mistake: At the court, Bender is shot in the back by the Professor leaving multiple holes but later when he is talking to the little girl all the holes have gone.
Continuity mistake: When Bender and Santa are destroying New New York, we see an outside shot of Planet Express with the Professor's titanium shutters covering the entire building. But when we see everyone huddling on the couch, you can see out all the windows. (00:19:35)
Continuity mistake: The elves' hats fix themselves in the shot looking from behind the sack towards Robot Santa, then become ripped again as they run off. (00:05:45)
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]