If prosecution can convince the jury that Hacking operates as you (and SpottedOwl) have indicated, so much the better. It will strengthen the case for the most apparent motive being a motive for murder.
They may be able to imply...but if he pleads guilty but insane, he can be examined by the prosecution chosen psychiatrist..Narcissism isn't insanity in that they know what they are doing is wrong...
He will know all the tricks and talk about"voices", I fear!