I've had dream about hiting punching kicking doing everything I can to hurt my girlsfriends ex boyfriend but nothing seems to hurt him its like he is invincible! I don't have that much history with him other than what my girlfriend tells me but for sum reason in my dreams its as if I no him and really want to kill him ! Any ideas on what it mean would be helpful thanks

Maybe he did something bad to your girlfriend?
Maybe you feel like he would come back in your gf's life and your girlfriend won't do much about it?.
Maybe he has that look and which it makes you want to hurt him?. cause my boyfriend wants to do that sometimes.
There's not much info to go on..

Probably because you don't like him. I find nothing odd about your dreams.

And maybe you think he'll come back into her life; thus, he's invincible.