For me it's a kind of but-for-the-grace-of-God kind of thing. I've sometimes gotten a sense that any of us are just one kill away from the crazy train. How many people kill someone, accidentally or not, and then stay sane? How many people get so near someone's death that the lines of blame are blurry, and then stay sane? Our ability to blame and punish ourselves is so immense. For a character who believes she's irredeemable, where does she go from there?
Plus, I love a good redemption story, but how do you make good if you were never sufficiently bad?