Do not warn the OW that you are telling her BH, she will tell him some crazy woman is going to call and blah, blah, blah. Do not tell your WH that you are going to tell the OBS, he very likely will warn the MOW.
I feel there are some situations that it is probably best not to tell, but those are few exceptions. In general it is best for all spouses to know.
Was it wrong for your WS and the MOW to keep their affair a secret from you? I would imagine most would say "yes". Now that you know, does it become okay to keep the affair a secret from the OBS? I don't feel that it does suddenly become "okay" to keep it a secret because you now know. It is still wrong to keep the secret from the OBS. And, the fact is, most WS's aren't going to suddenly confess. They are going to try to keep it a secret and are hoping that the OBS feels it isn't "any of their business" to tell their BS. They are counting on that!
Personally, I don't feel it matters what your motivation is to tell the OBS. Revenge, stopping the affair or a genuine concern, the end result is exactly the same, the OBS now knows the truth of their very own life and marriage. To me, that truth for the OBS is the most important thing, not your motivation for telling them the truth.
eta: it does matter that you are extremely kind and compassionate towards the OBS and in how you tell them, though, remember their world is blowing up and it isn't their fault what their WS did. BW (me) 50ish FWH 50ish
Married 34 years, 3 children
d-day 3/10 LTA (4 yrs./fucking & flirting)
"Oh, why do my actions have consequences?" ~ Homer Simpson
"She knew my one weakness: That I'm weak!" ~ Homer Simpson