Continuity mistake: When the everyone is singing and the Devil Robot is taking Bender down in the Lift, we can see that the back of the lift is grated in one close up shot, but in a far away shot it is a solid cube with no grate. (00:18:15)
Continuity mistake: During the Robot Devil's song, Fry and Leela are going down a slide. At the beginning and end of their verse, they are shown as being on the same slide. While they are singing, they change positions several times. (00:17:50)
Factual error: When Bender is dipped in the baptismal oil the oil level in the barrel doesn't change; which it should because of the Archimedes Principle. (00:10:20)
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]