Speaking strictly of those of us who are victims of infidelity, being unfaithful is IMO an inherently selfish act. But does being selfish equate to narcissism? Being as I hate putting labels on people, I really can't say.
As far as them getting worse as they age, my cheater former father in law certainly did. What was perhaps charming when he was in his 40s and 50s got disturbingly creepy when he got in to his 70s.
I think that many of us witnessed the "mask drop" including "shark eyes" and other traits. Again, speaking only of my own case, I do think that my ex worked very hard on being the sort of person she wanted to project. The loyal, loving wife. Pillar of the community. In many ways it was perhaps a rejection of the person she saw in her father and sister. She held that more or less for over half her life - nearly 30 years. Was that the person she actually was day by day? No - but close enough that for me it made no difference.
I knew all along that my then wife was selfish and self-centered. She was also charming, capable of being sweet especially when it served her purposes.
But I also think that infidelity is like a drug. The high that they get when "getting away with it" causes them I think to over-correct away from the person that they were trying to be, well past the person that they are and into a monster. Those who stand in the way of their fix are crushed.
Can my now ex build herself a happy life? I really don't know but do think that is beyond her now and perhaps always will be. A friend of mine was an OW who blew up a family. They're still together nearly 20 years later. Superficially she is happy but I also know her to be insecure and constantly feels like she is being judged. Another friend of mine cheated on his wife multiple times and has few regrets but is now terrified of aging alone. Oddly those two friends have been some of my staunchest supporters and are deeply offended by what my ex did.
There's a quote in a favourite book that is actually talking about evolution, but I think it applies here. “Did God make them change? - Certainly yes - but do they also change themselves? That is the question.”
On BD H52, W50 T27, M26 S21, D23 BD-9-Mar-16 D-15-Jan-18 Final-19-Apr-18 I am a storyteller. The story may do you no good. But a story is never for the listener. It is always for the one who tells