I think you should stick to the main point: the girl genuinely being redeemed. If that is indeed the case, then the guy could develop second thoughts as to whether killing her is the right thing to do. As he struggles with this, it takes time and they get to know each other better and better. He keeps trying to find out more, she keeps trying to convince him that she's good.
Are you planning to insert eroticism into this? That could be a thing as well. Perhaps the classic fairy tail trope that true love can break any curse?
As for how it should end, I'd say do what you feel makes sense.
