Whilst I would generally concur with the idea that serious diagnostic terms are bandied about on here far too loosely, I think to write armchair diagnostics off as rubbish tarnishes the hope for victims of this sort of person that their experience is valid.
To contrast :
There's a woman at my work who is not a nice person and is rather unstable with it. She would probably benefit from long term psychotherapy.
I don't consider her a sociopath. Just vulnerable, angry and damaged.
When I first came on Mumsnet, I had NEVER heard of NPD, the physical and mental relief I felt when I read up on it I cannot begin to describe, I was just like "oh my god, it's her, it's her!"
Deeper than that though, there are a few NPD traits that don't fit, but psychopathy traits that absolutely do. Dark Triad mentioned here was very helpful.
I've spoken on this thread about her plausibility before and one thing she's very well versed in is psychobabble speak and pseudo spiritual bollocks. Her professed interest in "that sort of thing" gives her the false impression of someone with emotional intelligence, but it's academic, book learned, catchphrasey, as opposed to intrinsic.
It's just another mask, just another deceit. She is an abuser, it is the core of who she is. The only person who has ever got it is her XH, and, in the breakdown of that marriage she cast herself as the victim there too and was literally believed by EVERYONE except me, who due to her constant abuse, saw and achieved insights into the marriage that other people were blinkered to.
I don't make light when I refer to my sister with serious terminology is what I'm trying to say, once upon a time, as a teenager I could have said "she's just an arsehole and I don't like her"
Nearly 20 years later, after decades of her behaviour, it's very clear to me that it's not just a personality clash, and there is something deeply wrong with her.