It's better to know, of course it is, I don't think anyone is saying they aren't glad they found out.
For me it's all about the motivation behind it. Telling someone because you genuinely care about them and can't bear to see them hurt is one thing.
The OP wants to hurt the people who hurt her. Completely understandable and no one would argue that it isn't deserved.
But the affect on the husband is seemingly viewed as acceptable collateral damage. It isn't. This is going to change this man's life forever. It will change him forever.
Betrayal destroys a part of you that you can never get back.
He will find out, he very probably has his suspicions already however hard he is trying to ignore them. But to find out like this? Horrible.
Mainly because it's just so humiliating, but also because it makes you feel a complete idiot. You're marked out as the last to know, you didn't find out yourself, you needed someone to tell you.
It's also pretty fucking miserable to know that the person telling you doesn't give a shit about what happens next. They just drop a bombshell that will completely obliterate the life you know and then disappear into the distance without looking back.
Not sure I could take any pleasure form that.