Hi! Speaking as someone who comes from an abusive parent, this is likely Seiichi struggling to accept that we was right. He was right that his mother was a bad person, on top of the fact that he's uncovered a repressed memory in which is mother tried to kill him.
However, throughout the entire story, she's continously gaslit him. It's to a point where he doubts his own memory. That's his mommy, that's the parent he's spent the most time with, your parents are supposed to protect you. It's hard to accept that you were abused, especially if they never put their hands on you.
But his mother is gone, the threat is gone, right?
So whats the issue?
The issue is that even if the danger is gone, your body is still in survival mode for months or even years after the fact. He's having what I call a 'facts vs feelings' response. His facts, his brain, know that he's okay. His brain knows that mommy is gone and she can't hurt him anymore. Everyone knows what she is now, and people will protect him now, right? He knows he shouldn't love his mother anymore. His feelings or response to the danger being gone are telling him otherwise. His inner voice is now his mother telling him that he isn't good enough.
Seiichi isn't a psychopath, he's an abused child struggling with what he now knows.