You should not say anything, now or in the future.
Right now, there is the practical matter that you are hoping that he keeps his defenses down so that you can position yourself better for the divorce. That is, you keeping quiet, right now, will directly benefit you. I might recommend that you not even bother with the full 180... be pleasant, be happy to the best of your ability... so that you can spring the divorce on him as best as you can. Think of it in terms of "The Godfather": Keep your friends close, keep your enemies closer. In this sense, also, your STBXH will likely use everything that you say against you. So, say very little.
Long term, I don't recommend that you say anything either (this was one of my biggest mistakes).
First, your biggest priority should be your children and telling them the truth about their father will only hurt them. You should do what you can to ensure that they don't feel like the divorce is their fault. Make sure that they feel safe and secure, but don't tell them the truth. Let them figure out that their father is a piece of crap on their own (hopefully, he'll pull his head out of his arse and they won't figure this out about him).
Don't go out of your way to present your STBXH as a good person either. I did this and I recently saw another SI member did the same. The kids found this confusing; they would ask "if he's such a good guy, why are you divorcing him?" Mostly, just don't comment on him around your kids.
Second, telling everyone what he did will make you look like the victim. While you are genuinely the victim (he cheated on you, after all), it's just not a good look to try to convince others that you are the victim. You can tell one or two very close friends or family members, but that's about it. Victimhood is like nicknames... the unwritten rule is that you don't give yourself a nickname and you don't make yourself out to be the victim -- let others do that for you.
Other than that, you need to put him in the rear view mirror, focus on yourself, and focus on re-building your new life with your kids. (this last sentence is the 180 that you should be doing).
p.s. I'm so sorry. This all sucks and it's not your fault.
Me: BH, age 48Her: WS, age 45 (multiple EAs and PAs)D-Day: August 30, 2016
Diagnosed with depression in December 2016, which was primarily caused by my xWW's affair and associated emotional abuse.