Qwerty, I couldn't find your original thread about when this started, but I thought he had just broken up with his wife/girlfriend. He probably didn't have enough time to really get over her. This in no way excuses his actions, but it points to why he did it. You guys had sex so he had no need to confront his feelings for her after breaking up. When this started you said that it was only sex and that you were just going to enjoy it while it lasted. I'll also let you in on a little secret.....men will have sex with someone they hate. I have specifically fantasized about having sex with someone who I hated. Not because I wanted to change my perception of her, but because it just seemed hot. I don't know why and I can't explain it in a way that will seem logical.
It seems that your biggest mistake in this situation was allowing a relationship to form instead of just enjoying the sex as you said you would. By entering a relationship with him you became a rebound relationship. I'm sure that if your were observing someone else doing what you did you would have advised them to stop before they got hurt. It never looks the same when you're actually IN the relationship though. I'm sorry that you lost a friend and a boyfriend. Hopefully you will treat this as a learning experience instead of beating yourself up.