What did I do to deserve getting blamed for him cheatin on his girlfriend?

You didn’t do anything wrong. You were in a bad position - clearly you can’t go to her and say “look what your boyfriend is doing” so you had to say nothing.

The only thing I would say is ask yourself if you made it clear that he needed to stop. If you didn’t then you encouraged it. If you asked him to stop, then you couldnt have done anything else (and in that case he is a weak weak individual).

However, don’t take this too hard on yourself. Just remember, she knows what he tells her, and if he tells her this, she is likely to believe it (she probably felt love to some degree for him).

The best thing you can do is to just say when she yells at you or he screams at you “why are you calling me… you knew you had a girlfriend”. If he keeps calling, keep hanging up. Play it cool, avoid the drama. He will come off looking like the psycho loser.

And as to answer your question as to why. The guy is a guy who thinks everything should work out for him. It isn’t going his way, and he is having a hissy fit and wants someone to blame - he is too weak to look in the mirror. He was too weak to leave her (he couldn’t have been happy) and he was too weak to approach you in a respectable manner. As a result, the reason boils down to one word. Douche - which is what he is.