Kate: "YOU KNOCK HER! YOU F*CKIN' KNOCK HER! YOU MADE MARIA BLEED, D'YOU HEAR ME!"
Maria: "KATE STOP! PLEASE! He doesn't even deserve that... Don't.. YOU'LL HAVE TROUBLE! I DON'T WANT YOU TO HAVE TROUBLE KATE PLEASE"

This guy on the floor hit Maria.. Kate.. Just couldn't let this happen.. She had to avenge Maria..
If I can reassure you, after this scene, Kate listened to Maria, and didn't end her act. Not to disappoint her. She has too much respect for her, really.

I tried to denounce bullying at school. If can do that..
And if you've been bullied, or if you are.. Talk about it with people around you. And know that Kate and Maria are there for you too.