My friend ruined my relationships, and it's making me start to resent him.
Ruined your relationships? How did he do that?
He was badmouthing them behind their back and that caused me to lose interest in everyone.
It's understandable that you are resentful towards your friend for sabotaging your relationships. You should use this foresight to take a close look at the value that the friendship brings you, versus the problems that has been caused. This will tell you if your friendship is worth keeping.
You don't understand. It's like talking to a brick wall. I'm done with you.
If you cut people off so easily, you will end up with no friends even if someone else isn't ruining your relationships. Maybe you should try talking to people patiently, including your friend or the other people you were interested in. You might be able to talk things out and build stronger relationships.