Suiyan
well like they said not been cruel to your ennemi it's been cruel to yourself! even though she was just a small an innocent child! i think if back then he took the girl in and raise her maybe it wouldn't turn out like this or he could have put her secretly undersurveillance so that she won't cause chaos or harm!
This reminds me of a vampire series where the MC had the opportunity to kill the vampire progenitor when he was just born. She couldn't bring herself to kill an innocent baby & suffered for hundreds of years because of him. At the end her consciousness went back in time with the resolve to kill him. She "forgot" everything that never happened. The god that helped her (who she no longer knew) later asked her if she was happy with her life, and she said yes.