I wish my late father's OW had just randomly walked up to us in the street and punched us all hard in the face.
Not least because it would have hurt less and had fewer long term implications. But also becuase then people wouldn't pull 
faces at the concept that I was, and more than 30 years later still am, incredibly angry with her. With pleanty of residual ill feeling and contempt left over, despite the passage of time. Vows, or no vows, I hold her responsible for her choices that she was well aware would have a painful and negative impact on other people, even if she had never met them.
But I am glad her husband knew before we did.
Becuase at 16 I would have had no hesitation in building a case of moral justification for telling him, and had few inhibitions about him becoming the collateral damage of my return fire.
We lost so much. The harm caused was long term, unfixable and has left an enduring sadness. I can't even mourn my father without it becoming so horrendously complicated and tangled up that it just becomes easier to pretend he hasn't died.
That's enough to be going on with. I think all the regret, the pain, the loss and "if onlies" would be harder still if I were left with residual guilt due to a strong suspicion I might have used another of their innocent victims as pawn in a game of pay back.
The person who tells him really needs to be somebody who has solely his best interest at heart. He hasn't been afforded any humanity, sympathy ,or consideration from his wife, or your husband. At this juncture, given the potential fall out, the very least he deserves is knowing, once the dust has settled, that when somebody handed him the equivalent of life letter bomb, they were doing it becuase their motivation was all about how much he mattered, counted and deserved better. He needs to matter enough to be told without an underlying agenda, rendering him once again a very much secondary consideration.
32 years on and I recognise the pain behind your words like it happened yesterday.
I'm so sorry you are going through this. If there were any way I could point to a release so it hurt less, I'd do it in a heartbeat. But I don't think this plan, how ever understandable, will give you any respite. And in the longer term may lead to one of those hot flushes of shame, when you suspect you essentially threw another human being under the bus for the sake of the "take that you motherfuckers" feeling you got from it.
You aren't your husband. You aren't the other woman. The throwing of another human being under the bus for the sake of what felt good at the time is their hallmark. I think it is a distinction they deserve to keep all for themselves.
another love.
I'm so sorry you are going through this ringer. I wish the club for people knocked for six by other people's affairs was one that would shrink, rather than keep on getting bigger.