@Grenzoocoon Well to be fair we do not actually know how many people they have killed and under what circumstances. Also killing them won't do anything for the crying children. Revenge is effectively useless and punishment is equally pointless. He specifically said if she stopped doing bad things. Killing her would only guarantee that she stopped. If you can get someone to stop killing then they are no longer a problem.
Whether you can get them to stop forever though is another matter. People have reasons for doing things after all. She for instance was just doing what her kind does and feeding. Its not like she's preying on her own kind after all or going against her ethics. Basically she's not a "bad person" by any standard you could reasonably apply to her.
If she straight up said she wouldn't stop or showed that she wouldn't somehow, he would have killed her. He stopped because what he saw was not a monster, but a frightened person. So he gave her the option of living in peace if possible.
All in all thinking about past victims is a shit argument for anything going forward. All that should matter is whether they will continue to be a threat. If i knew a murderer would never do anything wrong again i would be totally ok with letting them live normally. There are of course societal implications and the issue of being unable to tell the future but on a 1 on 1 basis i would at most fine anyone for any crime if i knew they would behave from now on.
Is the hero a little naive? Perhaps. He could also be a good judge of character. We don't really know. We can tell somewhat that this succubus is not a real issue if you handle her. I would say if anything she's a little too vanilla for my tastes.