Hate is as strong of an emotion as love. Maybe seek help for that as it is not healthy especially when your hate is towards the wrong individual. Your bf is not a helpless little child, he consciously allowed it to happen because he wanted it. If he at the time had any respect for you he would've set the other person straight from the get go but he was looking for someone to cheat with too. The other person doesn't owe you loyalty and fidelity but your bf did. You decided to stay and reward your bf by doing that so now it is on you. It was never the other woman's fault. He will most likely do it again with a different person or the same especially if there were no consequences for his actions from you. Doing the same can help you feel like you have your power back, plus he should be able to take what he gives but most likely won't. It will level the field for sure. I know that I would engage in payback and I have because two can play the game.