I agree that knowingly becoming a mistress is a really rotten thing to do, but it doesn't necessarily mean the person is evil.
Often they are very sad and a bit deluded, with some major issues that mean they cannot see how to stop themselves, or why they ought to.
Sometimes they are really very desperate and the fact someone is already married isn't enough to make them stop - they may feel as though this 'love' is their only chance at happiness, because they've never had that experience before.
They might not know how to deal with the pain of denying the relationship, once it has begun to develop.
I'm not excusing the behaviour but am trying to see how and why it occurs.
Obviously there are some really screwed up people who deliberately go out to hurt others, and seem to enjoy it but this isn't so usual I think as the lonely person who just has no clue what is the right thing to do - and may not even have any women friends so knows no sense of sisterhood.
I have never really understood 'sisterhood' stuff as it seems discriminatory against blokes, somehow, and a bit weird - but then I had poor relationships with the women in my family.
I would never want to become involved with someone attached, not now that I am aware of what can happen when someone does this. But I think often the OW is a bit irrelevant - even the stick analogy is a bit weak, in that the bloke wanting to beat the kid will just go and pick up another stick or even take off his shoe and use that instead...iyswim. He's still the bigger bastard.