How do I deal with shady ex?

I was dumped by my ex 3 years ago, and honestly I was heartbroken. Afterwards, I learned from various sources that he had actually been a very shady boyfriend, not just to me (turns out he was still with his ex when he asked me out, and told his other friends that I was the one who went after him, making me the home wrecker). I found out later also that even before he up and dumped me, he had already been hitting up other girls and was getting ready to already have "the next girlfriend" by the time he brought it up with me. A few of the girls he tried were actually a group of MY close friends that I introduced to him who told me how shady he was about it.

Since then, disgusted, I tried to have as little contact with him as possible for the past few years in college (which has been hard since we have many mutual friends). It may sound harsh, but I just didn't find any value or joy in keeping a hurtful person like him as a friend. Other ex's and I have generally been good about remaining close platonic friends after breakups, but this ex just had no intentions of caring - he only wanted me around afterwards if it would make him feel less lonely, all the while acting as though I should still be affectionate, but only until he had other appointments. I didn't need or want anybody like this in my life, so I kept my distance (I was less than friendly, which helped) and I've moved on.

Every now and then, I run into him at parties. Even now, he acts the same way. He greets me very nonchalantly, like we are still buddies, and if my boyfriend is not there, he'll try to have conversations. He texts sometimes after seeing me at parties, trying to be witty and strike up conversation which I shoot down by being blunt or brief. On the other hand, if his girlfriend (which was a girl I knew) was around, he made a point of being affectionate with her while I was near them, and I am not mistaken when I say he is looking at me when he does this (it's happened many times).

It used to hurt, but seriously I am really over this guy, and now more than anything I just want to know what the heck he wants when he does these things. Does he think that it will still hurt me? Or, I doubt that he is trying to be my friend again, but other opinions have said that maybe he just wants to be on good terms again?

Also, I've been getting questions from other girls that he's been flirting with (yes he's doing it again, and it's unclear to everyone what's going on with him and his girlfriend) asking "can you tell me about your ex, I've heard he's kinda shady" - how do i/should I even respond to that? I do know a lot of bad things about him, and I'm aware it's best to just keep my mouth shut, but what is the best thing to say?
How do I deal with shady ex?
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