Most of them want to be God, so they decide what is right and wrong according to their desire. So, in a way, most of them believe they are okay -- by their own moral standard. The sociopath tells himself he is doing you a favor by defrauding you so that you might learn a lesson. The cheating wife tells herself that some imagined slight justifies her actions.
If you think you have your own version of right and wrong that conflicts with God, then you are one of them.
I think that, first of all, evil (like good) is a relative kind of thing. We think of killing a person as an evil act - but not if we are killing someone who is about to (for example) press a button that will lay waste to an entire city. (Then again, what if that city had a population consisting entirely of really, really evil people and, by killing the person who we originally thought was evil, we unleashed thousands upon thousands of really, really evil people?)
Second, if someone realizes that they are evil, then this implies that they have a functioning conscience and, since one of the characteristics of an evil person is a lack of a conscience, or a lack of awareness of a conscience (a consciousness of a conscience? wait, I'm confusing myself... start again...) The point is, if a person is aware that he is evil, then that is an indicator of his potential for good.
Third, I'm not aware of anyone who is 100% evil (or 100% good, for that matter) even though there have been some in human history who have come awfully close to it. Like I said, it's a relative kind of thing. Compared to, say, Mother Teresa, Hitler was one evil SOB. But compared to Stalin, Mao or Pol Pot, the difference is so small as to not make a difference at all.
We all - ALL - have elements of good and evil within us. That's part of what makes us human.