In my case I hated completely the OW who deliberately pursued my partner. Her relationship had broken up so it was a revenge thing to make her ex sit up and notice. Pathetic.
We had a lovely house bought together, I had 2 young children. My partner had a good job and a good income. She wanted what we had, she had a low wage and no money. Little did she know that 3/4 of the property was mine.
She did not love my partner, it was all obvious to me, transparently so. My partner had low self esteem though and was always looking for admiration and validation. The OW identified this and made her moves. Initially I thought she was visiting us for support, which we offered in spades [big mistake]...but then it turned into stalking us, turning up everywhere we went. P was flattered, I just saw a lunatic.
My partner was weak minded, and deluded, liked thrills, excitement and drama [so OW fed into all the vulnerabilities] and partner actually thought OW was genuine and in love. Just couldn't/wouldn't see the reality of the situation despite me trying to explain what I was observing.
This is an otherwise intelligent person. I watched it all unfold in horror.
All I saw in OW was a predator, a user, a selfish manipulator, a calulator, a gold digger...and she got what she wanted. I was crushed, my life was awful for a long time. Made a new start as soon as I could though.
Partner saw the light after a few months after which it was too late. We tried to make another go of it but of course it didn't work. I didn't hate, I felt pity and sorrow for the way human beings treat one another and how weak some people are.
[Feel better for letting that out...many. many years later and I have absolutely no feelings for either party, two people at the mercy of their genitals who think happiness lies by sweaty, guilty couplings in someone elses bed. What a sad way to live]
It all showed me ultimately that we weren't right for each other, it happens, and I was glad to find out sooner rather than later so OW did me a favour in the end.