Narcs lie for one particular reason - to create the reality in which all the deeds of theirs are justified.
They don't know one thing, though. They are not able to comprehend their faults and sins because of pain that devours them from inside as if admitting they did something wrong might cause even more hurt. Of course, that's not the truth.
I am far from defending people with NPD, for most of them are who they became as adults due to their own choices (or lack of choices). But I perfectly understand that the pain they live with is unbearable, and no one who did not experience it has the right to assess that he or she would behave differently if he or she would be in narc's shoes. No, you have no idea what you would do, feeling what they feel.
Unfortunately, the problem is these individuals have no idea what they feel. They are as far as possible from their own interiors. And by denying wrong deeds, they are even further. It is hard to convince most of the people that confessing sins may help because, according to current social discourse, flaws and weaknesses indicate we are less, and no one wants to be less, especially narcs.
It took me lots of time to solve this problem: why denying hurts us? But it's simple. To deny the truth, we usually use cognitive dissonance. By doing so, we became personal adversaries of our own bodies; and we start getting crazy.
Cognitive dissonance is a bitch. Lying, treacherous, deadly bitch. Only, in case of people with NPD, it is a total game-changer; to be exact - brain-changer. Narcs believe that what they feel at the moment, and what they think, is the one and righteous judgment in the Universe. I was raised by the narcissistic witch; thus, I know exactly how it looks in practice.
"I go to church, and I am a catholic, but, in this case, I think I didn't do anything wrong because I feel so. Yeah, having an affair with a married man, having a baby with him, and not telling him about the pregnancy sounds not right but not in my situation, for my situation is different and extraordinary, and you must accustom my beliefs, not the doctrine of the Church".
You look at this person when she says something like this, and you think there is nothing that can help her. She is just lost since she has no hint that things can be otherwise. And even if she's your own mother, in most cases, it is better to run from her to heal wounds she caused within years.