Even if you are angry and you have reason to be angry, that doesn't give you license to be cruel. We have to take responsibility for ourselves and the way we act.
If you tell somebody enough, "You're stupid, you're not worth it, you can't do this, you can't do that," then how many times do you have to hear that before you believe it? The power of words is immeasurable.
I promise you, if you reach that point where you are not afraid to figure out who you are, what you want and what your values are, you will not fear that anymore. You will not do it, you will be smarter.