I need some insight in friendships?

So, I have this colleague , who is extremely selfish, and tends to treat people very rude. Now I know that this is mostly because of his low self-esteem, which he finds impossible to repair without subduing others. The problem is that, every time I try to help him, I find myself being betrayed and used. I know that most people would just choose to ignore him completely, but I can't help but feel guilty and selfish myself if I don't at least continue to try to make his life less painful. But I still can't accept him using me just to satisfy his moral needs. I'm at a loss...