Some of the stereotypes you mention though, Something are not on this thread. And I have often said that it doesn't help our understanding of infidelity if we paint the protagonists in one-dimensional terms.
I don't think this is about sisterhood either, but I do think it's about humanity and treating others as we would like to be treated. Just because you made that woman no promises, it doesn't mean that you were absolved of your responsibility to behave with integrity to another human being. Causing harm to a stranger is still wrong.
I also think you are demonising this man's wife and seem indignant that she was angry towards you. using your analogy, perhaps you need to imagine walking in her shoes to have full empathy with her position. She might have all sorts of flaws in her personality and character and it certainly doesn't follow that all betrayed wives are "angels" no more than any of us are, but she was a human being who was hurting terribly and whatever her faults, didn't deserve to be deceived.
Her H does sound like a weak man, especially if he was hiding beneath her skirts and alleging that she was making "threats". What you were also told about their marriage, by him and the people who really aren't friends at all, might be very different to the reality.
No-one really knows what goes on in a relationship and how people present their marriages to their affair partners is frequently jaundiced and contaminated by another agenda. You say he was weak and he was evidently supremely selfish - who knows what she had been putting up with for years? Infidelity never happens in a vacuum, that's for sure. His weak character and selfishness would have manifested in their relationship repeatedly, before his affair.
However, as I always say. Good people have affairs and good people do bad things, for a time. If it is isolated to a single bad period or relationship, it doesn't have to define a person. If it becomes a pattern however, it might.
I have a friend who was once in your situation and what helped her was to take responsibility for her own behaviour and to see how her own actions had contributed to so much pain. She had been saying some of the same things as you; claiming no responsibility as the single woman, criticising the wife and being rather self-indulgent about the man's weakness (it was also just about the worst thing she ever used to say about him, so I had a wry smile when I saw your post
).
Once she'd started to see his behaviour more clearly and could more accurately identify that actually, he was selfish and manipulative, she was able to humanise his wife more. Once she'd done that and was able to take responsibility for her own part in this, she was able to detach and walk away, resolving never to get involved in a relationship like this again.