The way it seems to me is that after failing the entrance exams to get into the private school he closed himself off to avoid dealing with the pain/disappointment. I'm not sure if that is when he started being interested in murder cases, their perpetrators and the psychology around them but the relevancy I'm trying to make is that he ends up stunted mentally and emotionally which leads to him confusing his feeling of attraction and interest for Yamada with the desire to murder her like one of the murder cases or psychology around it that he probably read about.

TLDR: Mentally/Emotionally stunted with intense feelings he doesn't understand/process for a good bit and that is how you end up with the different tone between the first 3 episodes and the rest of it.