The link offered above by Bill3 is a good one. Here's one I just found. It sounds a lot like your gf's situation, even though hers hasn't escalated quite this far . . . yet.

https://avalancheofthesoul.wordpress...relationships/

Your gf's ex is a seriously and, I think, permanently damaged person. Her involvement with him has zero chance of a happy outcome for her. Even if she decided she were in love with him and went back with him, he would wind up making her miserable. This guy is doomed by his uncompromising selfishness. On his way to self-destruction, he just wants to take someone with him . . . over the cliff.

I don't think he wants her, so much as he wants to ruin what she has with anyone else.