There have been a lot of threads on here with the exact same question, it crops up a lot.
OW usually fall into two categories, the ones who have low self esteem and will chase after a 'relationship' at any cost even if the man is married because they are lonely or whatever. Then the second category, the predatory OW who are massive arseholes who don't give a shit about who they hurt and do it for an ego boost, or to see if they can steal someone else's bloke because it makes them feel like the alpha woman. The latter particularly are nasty pieces of work.
My personal thoughts on OW - and OM - is that they are scum. They could choose not to be part of the ruination of another person's life, and that of their children, but they are so innately selfish that they will just do it anyway. I despise them with every ounce of my being, and judge them against the behaviour of decent human beings who would never ever dream of doing something so awful to somebody.
But, just to be very clear, that where an affair is concerned, the majority of the blame must always lie with the husband, or wife, who is cheating on their spouse. It is the worst thing you can do to another person, and is not forgivable. And as somebody who has been cheated on 4 times I can tell you that, although it is possible to move past it eventually and even reconcile with your partner and move into calm waters, there are some things which are beyond forgiveness, and cheating is one of them. In the heat of the moment when everything is exciting, and the affair seems so separate from wife/kids/home life like some sort of bubble, cheaters never stop to think about the reality of the fall out when they get caught - destroyed lives, heartbroken spouse in therapy, traumatised kids, having to sell up the family home, extended family or friends who are angry and won't speak to the cheater etc Doesn't seem so worth it then.