Karl is what you call a sadist. Yes, he has never bullied anyone, but once he saw you, he had a feeling that would change

He had caught feelings for you, but of course, he doesn't know how to express them properly. So, he takes pleasure in bullying you. Literally

He throws you against the locker, and as you slide down, he pulls you back up by the hair, grinning with his piercing stare

"Wanna know how pathetic you look right now?"