Not everyone that cheats is a rat, they are human, flawed,selfish - and you were never going to know so they weren't trying to hurt you (their version).
I believe both of these things are true.
People do get caught up in the thrill and the "strokes" the affair brings them. There's not only the brain chemicals, there's the excitement of the clandestine meetings, etc.
Some people have no intent to hurt their partner at all. They cheat just because of how it makes them feel.
The days and weeks following discovery, however, tell the entire story about the cheater. All people are human, all people can be selfish. However, "flawed" is another matter.
"Flawed" is indicated by what the cheater does when he is discovered.
If he can look in the face of the pain and devastation he has brought on his spouse and his children, and continue his affair, he is "flawed",
meaning that he lacks empathy and doesn't care about his family to a sufficient degree to moderate his own behavior to avoid hurting them.
I like this analogy. I like to shoot guns. I like sporting clays, target shooting, etc. I get very good feelings from doing it, it is one of my favorite hobbies.
However, because of the danger and the hurt guns can cause to others and the property of others, I confine my shooting to areas which are designed and dedicated to allow shooting.
People who shoot in other areas, where people live, where people have other activities, are "flawed".
I decided that I would not be married and have a continued sexual relationship to such a person. That if, I knew "then" what I know now, the words "I do" would have not come from my mouth.
At that time, my belief system was very different. I was a church victim, believing that God's "perfect will" was for me to remain married, forgive her, etc. So I stayed in the physical home, lived in the same house, however, the marriage itself was quite over, and so was the real, spiritual "home" it could have brought to the children.
In retrospect, Dr. Phil was right when he said "children would rather be from a broken home than live in one".
My kids would have been better off with one parent who was trying to live a decent and good life, than with both parents, where one of them was "flawed" and bringing a completely ungodly influence into the home.