He thinks he can play you, act nasty, run off with the crazy ex, then when it gets old reel you back in. It's like a game.
My guess is he isn't enjoying his freedom as much as he imagined he would. He's gone from a comfortable home, you, his children, to shacking up with a fellow addict whose focus is not about him. Maybe he figures he hasn't burned his bridges. You show him that he has.