Moral considerations aside, I am genuinely curious as to why you would want to get involved with a married man. When you asked him if he wanted to date you, you knew that if he would have said yes it would have meant that he was a cheater, a liar, a worthless man. So, why would you want to date a guy like that? I really don't understand.
Anyway, I think you should tell the wife, but not for revenge. Tell her because she deserves to know. Just send her an email or something, better not to do it face-to-face because it would put her in a very uncomfortable position. Write to her something like "I'm really sorry to have to tell you but I think you deserve to know that your husband has cheated on you. Here is proof" and attach a photo if you have one, or screenshots of naughty messages between you and her husband or whatever proof you have. Then don't contact her or her husband anymore.
If you aren't able to tell her without feeling like you are getting revenge, then don't do it.