I like that. As for the survivor, perhaps he should try exploring other hobbies. If he’s living like that, killing her will only make him feel empty and conflicted. Killing her to save others certainly wouldn’t make him just as bad as she was. It would be destroying an embodiment of evil and it would be just. Perhaps he could find a way to prevent her from harming people without killing her. Killing her to satisfy his own craving for revenge, on the other hand, would make him just as bad as her killing people to satisfy her own cravings. It is the same action in both cases, but his motivation changes everything. If he can’t kill her impartially, he should find someone else who would do it for the right reasons.
P.S. I don’t generally don’t endorse any form of murder, stories are an exception.