This is an interesting discussion.
Sealynx, you may be on to something when you say that Ns may be lacking in brain development. Aside from the NPD, my mother also displayed other signs of having an emotionally undeveloped brain. She seemed to be in a state of arrested development, as if her brain stopped developing past the age of ten. The baby talk, temper tantrums, even the movies she would choose to watch (Disney animation over anything complex). She always seemed like a child.
Hops, I also agree that it may be a form of insanity. The hard part for me (and I'm sure for many of us), is not knowing the origin of the trauma that was the trigger. My NM tells the story of a fairytale childhood, being an honor student, popular, etc. Some of her siblings have told me that she was a "problem child," who failed in school, and was abusive to her siblings. She has six siblings, who all grew up healthy, happy, and normal. I've never seen any evidence of M being abused by anyone in her family. If anything like that happened, no one is talking, so I've been left to try and piece things together on my own, and I'm drawing a blank.
Maybe some Ns are just born mentally ill. My mother does seem to have the capacity to love inanimate objects, like dolls and teddy bears, and also pets. Things that offer unconditional love don’t talk back. I've NEVER sensed that she loved me, and never saw that flicker that Hops speaks of.
M told me when I was a little girl that she rushed to get pregnant, because she knew that if she gave her mother her first grandchild, that she (M) would inherit more money. So I was created for profit. When my grandmother died, she split her inheritance equally between her surviving children and 13 grandchildren. My grandmother loved all of her grandchildren equally and never played favorites, was always very fair, which makes me feel that she wasn't an a N, nor did she abuse my mother. My NM, conversely, is currently using her will as a weapon, so I don't think she was cut from the same cloth as her own mother.
Some of us may never know the answer to that question: WHY? In my case, I think I just have to accept that it was what it was, and learn to move forward from there. I'd like to know the truth, but I doubt I'll ever find it.