Source: news.marvel.com

4. Iron Man

Iron Man, Avengers: The Crossing, Superior Iron Man - Marvel Comics

Tony Stark, everyone's favorite Marvel hero, couldn't possibly turn super evil. I bet you think he didn't do it once. And you'd be right. He did it twice.

The first time came during Avengers: The Crossing, in which we learned that he'd been a mole for Kang literally the entire time he'd been in the Avengers. Yup, he was always evil.

Tony went nutter-butters when everyone found this out. He tried to kill all the Avengers, but failed. A teenage version of himself killed him and replaced him in the Avengers. Buuuuut, when the Avengers fought Onslaught, died, and were rebooted into the Heroes Reborn pocket continuity, this was all wiped away, and classic Tony Stark was back in action.

The second time took place during the Avengers/X-Men event Axis, in which most of the Marvel heroes and villains had their moral polarities switched, making good guys bad and bad guys good. Iron Man was stuck in bad mode even after everyone else reverted, and he continued to use his evil Symbiote armor and predatory iPhone app to menace San Francisco.

Superior Iron Man is a good comic. You should check it out.