Throughout comics it is common for villans to not be mentally sane. From mild mental illness to full on sociopathy, the sepctrum is wide. However, wouldn't it be interesting if these traits common in villans were put on a hero.

A hero who's worst enemy is not within their rogue gallery, but within themself. Maybe they could show signs of being a sociopath themself, and so they have to make an active effort not to kill people as they would find it all too easy or even enjoy it. Or they might not associate themself with their darker traits, blacking out at random and beating criminals beyond what is nessasary just for the shits and grins. I don't know about you, but I think someone trying to fight crime while struggling with their own dark, twisted desires would be interesting.

That's kind of what they are currently doing with Killer Frost