He hadn't expected her to go back on her word. Not knowing what else to do, he could only continue to threaten her, to force her.
Eventually, she gave in. After he got her, he thought he would quickly grow tired of her and then be able to cast her aside.
But to his surprise, he didn't grow tired; instead, he found that he had invested his own heart into this.
He wasn't sure if he loved her, but he liked her, wanted to possess her forever—that was an undeniable fact.
Just for that little bit of liking, he could do many things, all to monopolize her.
Since his heart held no desire for this world, he had to hold onto this one spark of affection, he couldn't let it go.
If he let go, there would be no joy left for him in this world; he would be like a walking corpse, completely devoid of zest for life.
So, Ranran, even if you hate me, I won't let you go. As long as you have feelings for me, even if it's hate, I don't care.