Honestly, I blame all 4 for the miserable relationships they are in. All of them are not communicating their feelings.…
My opinion: when he was pressing the nail he wanted it to hurt. But not because he wanted Sei to hurt but because he wanted to care for him. And he was disappointed that someone fixed the problem for him. He knows that the only thing that keep Sei with him is because he was caring for him after the death of his parents. He has to be needed. He probably loved Sei but not like a lover. I don't really know what's going on in his head because he really hate physical touch so he's not even cuddly. So is he manipulating him to keep him or is he really attached to him?