How do you know this?

SWIM told me?

It is, as you noted, a truism many people believe in that "almost nobody is a villain in their own mind", and instead people have justifications, special pleading, and other thoughts that excuse them to themselves.

My spin, of course, is that those people make up stories like that because they are reluctant to admit that other people are less insecure and don't need to make up as many excuses for themselves.