Continuity mistake: In the shot where the note from Leela's file gets ripped off, note the small tear on the note. Then in a later shot when Fry is showing the Professor the note the small tear has disappeared but then later on the small tear is back again.
Continuity mistake: The hammer on Leela's blaster changes from smoothly curved to angular.
Continuity mistake: As Bender is stomping to stuff the whale down the sewer, when the rope gets to the pavement, the top where the cover is tied suddenly jumps from the level of Bender's head to below the spot where his legs and body meet.
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]