This is terrible to hear of. I'm
not happy to have correctly called this one.
I don't think the ultimate issue is whether the OP's behavior was so beastly that it warranted him snapping back with (such an unsavory) vengeance.
To me, the bigger issue goes back to a subject I've mentioned in the past: CHARACTER. Regardless of OP's transgressions, his reactions say distressing things about his own character.
A person can say a mean thing in the middle of an emotionally charged fight and then later (actually, immediately even) regret it and make a sober apology for it, both with the words "I'm sorry" and with reformed bahavior patterns. I have done this a few times with my own SO (and MANY more times than I care to count with my immediate family members; I get heated with loved ones). I don't think that's necessarily a fatal deficiency of character; it's acting an arse, being foolish, and being human. And then making amends and setting yourself straight.
Quite different is the situation described here. Before we knew of the man's shoddy recent behavior, we have a man who, within TWO weeks of breaking up with OP, attached himself to a new woman and had sexual relations with her.
Character warnings!
But let's give him the benefit of the doubt, just for argument's sake, and say that he wasn't on the prowl for a new woman. Instead, in his confusion and hurt over the situation with the OP, he drowned his sorrows with a random woman. Still a character issue IMO, because he is using another person to comfort his own loss, and the urge to so intemperately "drown" one's problems can be an indicator of deeper issues.
Now we have gained even more valuable information about him:
Now we know that when poked, he struck back with a vengeance. His behavior reminds me of a rattlesnake that a kid throws a pebble at and the snake's response is to rear back and sink its fangs into the offender's body. Whoa there!
Ultimately, the OP has learned--much sooner rather than later, thankfully--that this man does not handle his own hurt in anything resembling an appropriate fashion. How people react to things when the chips are down reveals much more about their character than their behavior during times of smooth sailing.
And the very bright side is that you no longer have to feel that you may have stupidly given up what could have been a good thing/good man.
It's actually a lucky break for you that you learned your lesson about beastly behavior in fights with THIS man...instead of with a man of genuinely good character who might be in the running for life partner.
Now, a little bit wiser, you can start afresh.
Good luck to you.