Continuity mistake: When Fry tells Bender to be quiet, because he is scaring away all the fish, the length of the line from his rod changes between the shots. (00:02:50)
Continuity mistake: When Leela throws the harpoon at the boot, the angle where the harpoon hits the boot changes between the shots. (00:03:15)
Continuity mistake: When Bender and Fry are in their cabin the cabin window change position between shots. (00:11:10)
Continuity mistake: When Fry and Bender are sleeping, the knob on Bender's chestplate appears out of nowhere. (00:11:20)
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]