@Kellon That's how deprived of love he was. If someone has been starving long enough they'll eat literally anything edible you give them and treat you like you are god (there are some exceptions, of course, but my point still stands). As he said, she's become the only thing that relaxes him and makes him feel satisfied, which means that this entire time he hasn't felt relaxed nor felt any satisfaction in anything he's done till this point. It may be a part of her calculative plans, but she is the only one that is actually showing him love and affection. Whether or not she plans on betraying him later isn't entirely clear, but it seems like she genuinely just wants him to become a better person through her efforts, even if her efforts are superficial. I mean, I think she kinda cares about him to an extent, but he did kill her in one of her past lives so it's hard to say.
@ninekuku I'm pretty sure she just wants an easier life and to have some freedom. She recognized that he just needs some love and affection in order for him to favor her and give her what she wants, so I think she's willing to forget what he did in a past life. I doubt she forgives him, but she also likely understands that it was partly her own fault for being stupid and, more importantly, not providing him affection as family when he so obviously didn't get any from their father. He's still not a good person, but her hope is that he will become one if she continues to shower him in love, and she can only do that if she lets go of him murdering her in a past life.