My girlfriend's brother stereotypes me so I am going to scream at him.
Why do you feel the need to yell at your girlfriend's brother just for misconstruing your nature?
Because I can't stand his bigotry towards me.
I understand that you are upset because your gf's brother categorized you, but screaming at him is a bad way to resolve the issue. Talk to him so that he understands his mistake.
Nah, I want to ruin the future relationship between him and my gf by showing her how much of a family nightmare I can be.
What he did was wrong, but now what you are doing is even more wrong. You shouldn't break them up, and instead speak with him about how he hurt you.