I like to start off as good and squeaky clean as possible, helping everyone, and everything, in any possible way. But then... slowly, the hero loses it.
He never gets proper payment, and everyone uses him. He feels isolated, and lonely.
Eventually, after all the combat he has gone through, he keeps having flash backs of all the horrible things he's seen and partaken in.
He cracks.
He runs through out the landscape, killing everything in sight, attacking even superior foes, as he does not want to live anymore.
He can't. He just can't. After all the slaughter, he just stops, and breaks down in tears in the middle of danger, and he remembers his parents... faint remnants of them, and he whispers these three words before the bullets strike him between the eyes:
"I'm sorry, Dad."