Kim Possible S03E04 - Bad Boy

Ron arrives at the opinion that "bad boys" get the girls. As he prepares to test this hypothesis, a mishap with Drakken at a villains convention causes Ron to become pure evil, while Drakken can't help but do nice things. Before Kim can realize what has happened, Ron takes Drakken's place and Shego is sure that this time she's hooked up to a villain with actual potential.