Sam Winchester - Supernatural
Some may know him as Sam Winchester, others as Evil Sam. He has been in the claws of darkside many times from a drinker of demon blood to a vessel for Lucifer. He seems to have been cured of his evil ways, but who knows what's still lurking deep inside his psyche.
Dean Winchester - Supernatural
Bad boy Dean is a magnet for bad. He became a demon to save his life early on from which he fully recovered, but Dean's worst incarnation of evil was when he was possessed by Michael who wanted to use Dean as a way to take over the world. Dean is back to being the Dean we all know and love, but who knows what other evil forces have him in his sights!
Faith Lehane - Buffy the Vampire Slayer
Faith was a slayer who wanted to be good but let her damaged background mess with her head. She let the darkside in because it was an easy out but she eventually realized that despite her former evil ways, she was meant to walk in the light.
Poison Ivy - Gotham
Ivy wasn't necessarily a good girl by any means, but she had a certain innocence about her. After barely escaping from Fish's Doctor Death, she turned much more evil after she downed some weird power juice. Armed with the knowledge that plants were her friends and creating a hypnotic "perfume" to get her way, Ivy has become one of the most dangerous villains in Gotham.
Arya Stark - Game of Thrones
Maybe Arya didn't go completely dark, but she's definitely teetering on the edge. And who can blame her after all she's been through in her lifetime? There is truth to the statement that Arya is a fighter and a survivor and maybe even made of steel, but she's done some pretty bad things even if she sees herself as a purveyor of justice now.
Theon Greyjoy - Game of Thrones
It's been tough for poor Theon. He can't be blamed for his descent into the the darkside. Ramsey changed him, and not for the better, but he's on the road to recovery after a violent past. After all, he is a Greyjoy. And a Stark.