You need to let it go. He will never have a happy ever after. Not really. No happy ever after included beating the wife to death, did it? She won’t believe you so you can’t help her. She will, sadly, need to learn the hard way.
Your only revenge needs to be living the best life you can. You can’t do that whilst you hold onto the past. Visualise dropping it, stomping on it, letting it go. When the negative thoughts enter your mind, push them away - do it with your hand if needs be. You have to work at it and at the same time work on you, invest in you and your future. He’s past. Move on.