I get the impression that this is going to be like explaining colour to a blind person.

Something that could cause another person serious distress while bringing very little to your own life.

Of course that's all subjective itself.

Subjectively speaking I believe that the level of damage an NPD causes to feed their ego is not warranted by what they actually obtain.

Cheating on your spouse because the person you cheated with is a 10 may feed your ego for a few weeks but it may also cause your spouse years of self doubt and trust issues.

But what I believe to be the real issue with NPD (and why it should be classified as a disorder) is that the type of ego involved in NPD is not the healthy ego of a 'normal' but a coping mechanism to hide a hidden internal pain.

Healing NPD should not be about the damage you cause to others but about learning to actually love yourself instead of pretending.