I kinda suspected that there might be the angle that she's into the other teacher. He isn't wrong that it's not right for him to be in a relationship with a high school student, but he's coming from a hypocritical space himself. In the end the only person who did nothing wrong was the introverted kid and she also ended up being the only one who survived, albeit with a lot of trauma that's going to mess her up for life.
Honestly, he got off easy by just dying, considering the mass amount of destruction and trauma he created. And plenty of other students ended up dying as collateral damage who weren't involved parties either.
Well. Off to the next monkey paw wish superhero.