Anonymous Conservative explains how similar behaviors stem from very different sources:
My view of the Narcissist is their amygdala is too painful when triggered, and their brain is not able to handle the stimulation of it. The narcissists I have observed would actually see their brains melt down when triggered, and it would manifest in what looked like incredibly unpleasant physical symptoms, almost combining a seizure, and the gastrointestinal upset and sickness of a major illness.
The psychopath is the opposite. Their amygdala is not there, so they don’t really feel fear. I am reminded of the character Hannibal Lector in the book Hannibal. At a critical moment, man-eating hogs are released, and rush toward Hannibal, who is holding FBI agent Clarice Starling in his arms. But the pigs move around Hannibal, because he feels no fear, and the pigs detect it. Although the scene is fictional, that is how psychopath brains operate.
Now narcissists, out of necessity, eventually hack their brains by using a false reality to shut off that amygdala-pain. They develop the ability to force their brain to believe something untrue, just so their amygdalae will feel relief and their amygdala will not turn on. I am quite certain it begins in childhood. As children however, I am not sure if they force themselves to believe an untruth, and that eventually becomes more more common as their brain finds it relieves angst, or if the untruth, when contemplated, is so relieving their brain cannot tell it from the truth. From their amygdala’s perspective, that would feel the same as when we find believing a falsehood irritating, and as a result we seek relief when we default to truth.
But once a narcissist develops this hack, now their amygdala’s influence on the brain and behavior is very similar to how a psychopath’s amygdala influences the brain and behavior. It is as if the amygdala is not there. The psychopath just feels nothing, while the narcissist alters their beliefs until they feel nothing.
Interestingly enough, he concludes that narcissists are more dangerous than psychopaths, because psychopaths are too clueless to be able to conceal themselves or their deeds very effectively.