I think this is such a complex issue that there's no way you can always call the OW a 'troll'. There's equal fault whenever adultery occurs.
I was the OW when I met my DP. He was unhappily married (aren't they all) to a woman who'd been a teenage friend. They weren't desperately unhappy - they were just more friends than husband and wife, and when she eventually came clean about never wanting children he felt robbed I think, because it was his greatest dream. Then she cheated on him - after which point he couldn't see any reason to continue being faithful, but didn't have the courage to end the marriage.
When I came along he was totally open about his situation, and so I knew I wasn't only the OW but I was one of several previous OW. I was young (only 21) and very stupid, and I desperately tried to get out of the mess I was falling into. We became heavily emotionally involved though, and I hated myself for it. Eventually we broke up because he chickened out of telling her it was over, and I ended up with depression and almost wrecked my chances of the career I'd spent my life working towards.
However, while I was busy getting myself together again he sorted out his life too - left his wife, divorced her and asked me for another chance. I tried so hard to say no, to get away from him, because I knew "If he's done it once ..." but I couldn't keep myself out of it and tentatively saw him again. We took things really slowly and he's had to work seriously hard to earn my trust. But we've now been together for years and we've got a baby, and we're really happy. Other people tell me he's changed, not in fidelity terms but his whole demeanour - he's content now. His ex-wife has remarried and is seemingly happier too.
He might do it again - but he might not. It's a chance I'm willing to take because what we've got is worth it. I do trust him wholeheartedly now. I wouldn't expose my son to the risk of pain if his dad was to cheat, so of course I trust him.
Would I do it all over again though? No way. I know that's like saying I wish my son hadn't been born, which of course isn't the case. But looking back if I was put in the same situation again with age and wisdom I would not repeat it. I have been very, very lucky. Am I a 'troll'? Some people will always think that, but the bottom line is that we both did something very wrong. It's worked out OK for all involved, and we all (including his ex-wife) know that the marriage was essentially over anyway, but it doesn't change the fact that it was morally inexcusable.
I do agree with MuthaHubbard's point that it's got to be better for kids to grow up with examples of happy adult relationships than to learn that marriage is about resentment, bitterness or even simply tolerating unhappiness. A dead relationship is a terrible example. Thankfully I have no experience of that particular situation.