Wanting to harm others helps the bully cope -- poorly -- with her or his own uncomfortable feelings of weakness or ineptitude. There may be a sexual component in this relationship, too.

I'm not so ready to take what your emaciated friend said about his weight at face value, by the way.

You are better off staying away from him. He needs help that you cannot give to him. Since you are in college, you might see if there is counseling available to help you deal with your need to dominate others when you see yourself as a "girl."