Yea, beating him up could possibly make the girl hate you alot. If hes the dirtbag you really say he is, keep your eyes on him at parties and stuff to make sure he stays in line.
Like grav said... this is obvious. You don't like him, you like her, he is standing in the way of you and her. KILL HIM. Does noone else see this easy solution!?! I feel like I'm taking crazy pills or something!!!!