Why does my ex blame me for everything when he slept around and was emotionally abusive?
No one wants to be the villain. It doesn't matter how horribly they acted, being guilty is a bad feeling and it makes it much easier to just pin that guilt onto someone else. If a person is emotionally manipulative and abusive, they've already got a lot of practice doing that. It's frustrating that people can ignore their conscience so easily, but unfortunately it is often only the people who are highly principled who own up to wrongdoings.
It would be best not to spend too much energy being frustrated about it. You know the truth, it really doesn't matter if this person continues deluding themselves or not. All that matters is that you've moved on and are in a better position after cutting ties. Don't engage in gossip, and if this guy tries to spread rumors that its your fault, people will see you taking the high road and you'll immediately seem more principled and trustworthy (unless they're biased beyond reason, in which case you wouldn't convince them anyways). Also, I'd advise that you keep avoiding this person as much as you can.
They may blame you, but you know what happened, so hang on to the truth and resign yourself to the fact that sometimes people throw the blame onto others because to do otherwise would mean having to face what a horrible person they were.