I don't think this kind of talk about narcissists is helpful at all. Cluster B personality disorders are to a great extent caused by childhood trauma- they're maladaptive responses, not some evil demon taking over the person in question. And not only that, they're incredibly unpleasant for the person suffering from them. Of course, you shouldn't expose yourself to bad behavior because of someone else's suffering, but you shouldn't separate that person from their humanity either.
I don't have any type of Cluster B disorder myself, but I have some friends who are, and the fact that half the world seems to think they're evil demons have caused them immense harm, so I think it's best to judge on a case-by-case basis.