I'm not going to judge you. What would be the point? It won't change your mind about anything or make thing better for the poor wife.
So I'm going to be pragmatic instead and deal with this from your perspective - which is the only one you can talk about with any certainty as you (or anyone on here) has no idea how the wife would feel in this situation because you (and we) are not her.
Are you hoping that an ultimatum to your OM will force him to leave so that you can do the same with your H? If you are, try it and see what happens. He may take you up on your offer (very unlikely, but it does happen). If he doesn't, what then?
An affair is in part so magical because it comes with a lot of the upsides of a relationship (starry eyes, sex, fun and laughter) without any of the downsides (no children, bills, etc). You know that waltzing off into the sunset with the OM and having a happily ever after is not really an option. Even if you both left your spouses voluntarily, the resulting devastation and practical concerns to do with housing, divorce, children and contact arrangements would somewhat taint your fairy-story. Are you prepared for that?
If you accept that you're not going to have a happy ever after, what is to be gained by telling the wife? Your motivation will be pure anger and spite. Regardless of who deserves what, remember that revenge takes something from the person dishing it out as well as the person on the receiving end.
You'd have to live with the knowledge that you are the sort of person who reacts with spite when hurt and angry. You'd have to live with the knowledge that you directly caused another human being intense emotional pain (because you make it direct when you tell her, whereas up to that point, the main responsibility is on the cheating spouse for causing that pain). Do you want to carry that burden? It can make future relationships harder because subconsciously the guilt can make you feel undeserving.
You may also have to face a situation in which the wife believes you are a psycho and goes on the offensive against you.
Think about it from this POV - if someone has behaved badly in a relationship with you, you couldn't possibly go around telling all their future partners about the risks because that would completely take over your own life. It's an unhealthy way to live being defined by someone else's past actions. A life well lived is always the best revenge.
What happens between the OM and his wife is their concern. If he is a twat in general, chances are he'll get caught out for something at some point. People generally do and usually when they least expect it. The same goes for you, of course, so consider leaving or fixing your marriage instead of seeking an escape outside of it.