From Nack's perspective, Mina (the bully) was an obstacle that reminded him of his own weaknesses. Since beating her allowed him to overcome some of his insecurities and issues (aka he felt better about himself) he could forgive her. It's easier to forgive someone when their transgressions aren't something that really eat at you.
Not that I'm saying this excuses Mina's behaviors, but at the very least, her attitude towards him is a lot more respectful
