What a tragic character. She is still a danger that has to be stopped but there's not much you can do to help the mind of such a deeply broken person while in a death game. It seems to me that her own Father began to rape her when she was a young child, maybe 4 or 5 years old. As he did, he would also beat her for not "growing up fast enough". The mother was aware of all this and blamed the child for the Father's attention, also beating her. It seems this went on until Mistuko went to High School. So at least 5 to 7 years. Over time, the Father would focus more on smashing her head - he clearly got off on extreme sadism.
One day, Mitsuko met a thug who she seduced and over a very long time managed to convince to kill her parents. Probably just by telling him the truth of what they do to her. As he did, she called the police having him arrested. And just like that she was free.
But she spent nearly her entire life knowing nothing but sexual and physical abuse. Her developing mind conflated this with the meaning of love and lust. And she recognized that she only escaped it by acting like her father - by lying to another man with words of love, then using him and hurting him.
This is why she has such a femme fatal complex. Her seducing, entrapping and using men isn't just a revenge thing, it's literally how she thinks the world operates. It's also why she targets men who remind her of her father. Reinforcing that she is no longer the prey, but the predator.
So for her, who's idea of love is intrinsically tied to pain and suffering, she was very confused by the anime boy, who offered genuine compassion and empathy. When he was hurt, Mitsuko began doing the only thing she ever knew could help - raping him. And when he said to get of, she took that as a rejection. Surely, if he really loved her, he would want me to hurt him!? And so, feeling righteous fury that this boy was just lying to her, she kill's him, and falls right back into the worldview she nearly escaped from.
It's not often you get child abuse and it's effects explored seriously in anime these days. Coming into this manga, I expected it to essentially just be violence porn. But the character writing is seriously top notch. Everyone's so different but 'real'.
I hope Mitsuko recognizes the anime boys genuine care for her before she dies.