I generally agree with your feelings about the lack of good in the player party but I do have to disagree with you here. I very firmly disagree that one can't turn away from past wrongs to be a "true" hero. You're right that the good done in the present doesn't cancel out the evil done in the past, but at the same time the evil done in the past doesn't cancel out good done in the present. I firmly believe that it's never too late to turn away from a dark path and do better, be better. You're not entitled to praise, acceptance or forgiveness for it, but if you're devoted to doing good and making the world a better place, then you can very much be a hero. Or at the very least, you can be a good guy.
Actually one bad deed can totally erase an infinite amount of good deeds, it doesn't matter if you want to change and be better, it doesn't bring someones child or husband or parent or whatever back. You will forever be a bad guy.