Why do people have to find ways to rationalize or justify their bad behavior?

I've known a couple of women who were involved in affairs with married men, and they both managed to find ways to justify their actions. One woman justified her affair by saying that the man's wife was self-centered and didn't care about her husband, simply because even though she was a stay-at-home mom, she still had interests and hobbies outside of her husband and children. The other woman, who was stuck in a miserable marriage herself, justified her affair by claiming that her boyfriend was just as miserable with his wife as she was with her husband! Don't get me wrong. I don't claim to be perfect myself. Indeed, I had a brief affair with a German man who was in town on business, despite the fact that I knew he had a live-in girlfriend back home in Germany. However, I knew from the get-go that my behavior was very wrong and I never once tried to justify it to myself or anyone else. Why can't some people just be honest with themselves and others and admit their behavior is wrong?

Because it would mean that they would also be admitting to their own character flaws - when the bottom line is: they know better.

Because it's human to do so.

Because if a person is doing something and they admit that it's wrong, they have no answer to "then why do you keep on doing something you know is wrong?" No one wants to believe they're a 'bad' person and wilfully doing something that they believe is wrong puts them in that category.

They may know it is wrong but they will never admit it to someone else.