How do I tell my boyfriend it's over without having him go completely insane?

I fell in love with the wrong guy. I admit that. He had a previous marriage that was falling apart and we started to go out. They weren't divorced yet and I didn't help them try to work it out obviously. But the thing is he lost a lot when he began dating me. Now he's not who I pictured myself with and we don't click. I need to break it off but he lost so much that I fear he's gonna lash out because of what he lost for me.

I was stupid for getting involved. I have been punishing myself for this. What I need to know is how can we part without a mess?