Problems with an ex wanting a friendship with me?

So I dated this guy eight years ago or so and just two days ago he messaged me on facebook and friend requested me. I was like wtf?? He said in the message that he wanted to be friends but we were never friends to begin with. Then he said that he would love to have his revenge on me but he is not going to. Then he said that he has a girlfriend for five years now and he can do stuff with her that he never did with me. He hinted sexually. Like I care. So I put it in his face and said I can do more stuff too. So to talk about what happened when we dated. He asked me out in class when I was in college. I didn't know that he already had two internet girlfriends at the time he asked me out. I found out later and told him he has to stop dating them. So he said he did but that was a lie because I found out he was still dating them and having phone sex with them. So I dumped him because of it. He cried on the phone. I said I loved him but I couldn't take the cheating that would not stop. He said that I was breaking his heart because I said I loved him. I said I know but I can't be with someone who can cheat and not stop. But my question is: what is his real motive here? Why is he even contacting me after eight years of being separated? We were only together for three months too btw. Should I even be friends with him? Is it a good idea to be friends or should I leave him in the past? I don't know what to do here. I have never been in this situation before.