Best thing is a Narcissist knows the difference between right and wrong, they just don’t care and think they are special and will get better treatment, therefore no insanity defense.
Because: A narcissist can stop himself. He just does not want to. A narcissist cannot be "treated" in the same way other mental disorders are treated, i.e. medications, talk therapy, group discussions, etc. Those methods are/can be used, but the narcissist usually does not agree to any such treatment.
A narcissist even believes that he is smarter than any psychologist, psychiatrist, etc. and often can outsmart them. A narcissist lives life only to play the game of outsmarting others, getting what he wants, and leaving others devistated in the dust. That is his need to manipulate and control all...and his environment. Again, narcissists...those who are so severly infliced with the negative narcissistic behavior are inclined to be immoral or criminal - and are very scary people totally capable of the worst crimes.