Favorite "evil" powerups?

As the title states, what're some powerups that, while useful, the hero cannot use in clear conscience? It could range from being too destructive to use, actively corrupting its users, or any sort of thing.

An obvious one, but the Black Suit in Spider-Man. While wearing it, Peter's abilties get enhanced, he can create his own web fluid, and can also make his own weaponry. However, the suit also makes him way more aggressive and draws out his worst qualities. Both Ultimate Spider-Man and Spider-Man 2 show this best in my eyes. In Ultimate, Peter goes on a joyride with his new abilities, until he nearly murders a mugger he hallucinates as the guy who killed Uncle Ben, which freaks him the hell out. SM2, meanwhile, has Peter becoming a shittier person the more the thing is on him.

Katara with bloodbending in ATLA. Aside from just looking disturbing, Katara didn't want to learn it in the first place, and was forced to use it by Hama to save Aang and Sokka. She then promptly breaks down crying, both because Hama won, and that she had to go against her own morals to save her friend and brother. The one other time she uses it, she instantly regrets it when she realizes she had the wrong target. It may have some positive usage, but it's still an ability that is far too easy to abuse and use for evil.