Strictly speaking, NPD is a psychiatric disorder. Unlike mental illness though, it's largely largely resistant to treatment, in part, due to the nature of the disorder itself. Most people whom are disordered feel they don't need help. In some cases of borderline personality disorder (closely related to NPD) there has been some limited success with cognitive behavioral therapy. This does nothing to treat the disorder per se, but does give someone disordered better strategies and coping mechanisms for things like self-harming, very common in borderline.
To someone who lives with someone disordered, I think it's really hard to see it as true mental illness, especially with narcissistic and antisocial. I'm heavily biased in that regard. I struggle to see my ex as anything other than evil - and I'm not being hyperbolic. APD/NPD leads to a particularly nasty individual extremely skilled at manipulation and capable of immense cruelty. What I don't often openly admit is that my ex was actually charged with and eventually plead guilty (to receive a reduced sentence) to drugging and raping his then 14 year old niece. This was a girl whose diapers he'd changed as a baby and often had rocked her to sleep. He blamed me for it, because after a decade of an extremely abusive marriage, I'd left him a few weeks before he raped her. I think this shows to what extremes someone disordered (APD/NPD) is capable of - no one and nothing else matters except themselves, what they want, and when they want it.
There's only one case I know of in which a diagnosis of narcissistic personality disorder has been used to some success - Brian Blackwell was convicted of manslaughter instead of murder but his sentence is still stiff. Because of the nature of the disorder, I would highly doubt many defence psychologists would put this diagnosis forward, even if it's an accurate one. If you followed the Jodi Arias trial, the State psychologist diagnosed her as borderline. The defence refuted that diagnosis but under cross was forced to admit personality disorder, other. Essentially, such a diagnosis can be fatal to a defence because while it is a disorder, it's impossible to separate it from the actual character of the person. I've said it's like cancer - it takes over the original personality. All of the cluster B's are 'known' for their manipulation and sometimes deceit. It's not a good dx to illustrate innocence, in my opinion.
Brian Blackwell - Wikipedia, the free encyclopedia
