Of course they are. They have no integrity. It would be just as easy to talk to you and tell you that something in your relationship isn't working but instead they choose to lie and cheat. To try and justify their behavior only allows them to continue to do it.
I think they are people who make stupid decisions. Sometimes they're bad, sometimes they're selfish, sometimes they're just cowardly. Being attracted to someone when you're in a relationship isn't exactly a rare occurrence, but then you need to make an effort to focus on the current relationship, or have the decency to end it before chasing new tail.
They may not be bad people but they are behaving badly. To me a bad person would be straight up evil. A cheater may have temporary issues that can be resolved. But I would never waste my time trying to help a cheater resolve their issues.