Simply because of publishing schedules, villains outnumber heroes by 12 or better. Most of them tend to be knockoffs, but what getting powers warped your moral compass? (I don't remember saying this before, but I might have.)
Brandon Sanderson and others have touched on this, and today I read the interview with the last Nuremberg prosecutor, who holds that war makes otherwise good people into murderers.
You could go full White Wolf and introduce a Morality stat--like SAN in COC it would deplete unless you took pains to refresh it, but that seems a bit too dark for me. (In honour of Traveller we could have a mustering out roll as the last one in character creation, and if you fail it, your character is a super ill aim. )
However, I'm a tad more hopeful than that. So you have a Heroism stat, but it doesn't fluctuate. It's just a roll you have to beat. If you don't beat it, then sometime during the session you get to redeem yourself. Or you can choose not to, and the target number ticks upward.