I know one diagnosed narcissist who fits the whole stereotype up and down, and I know a lot of other people with diagnosed personality disorders who are not the monsters society accuses them of being. I also know a lot of selfish assholes who have not been diagnosed with any mental health condition. I think to a large degree people use medical language to describe people who have hurt them or behaved badly, or just behaved in a way they don't like, because it reduces that person to an irredeemable, absolute monster. You can write them off as something inhuman, like a vampire, and absolve yourself for failing to recognize them, or their abusive behavior, or acting on it, or whatever feeling of guilt or failure is gnawing at you. P*do gets thrown around very casually the same way - If you can get your audience to accept that your opponent deserves the epithet then you can do whatever you want to the opponent. They're an absolute monster at that point.

It's not good. Medicalizing your opponent in an argument or whatever doesn't generally serve your argument. Yeah, yeah, fallacies, whatever, fuck the debate club nerds, but in this case the ad-hominem attack is something to be concerned about. And when it become a widespread way of shutting down someone's argument or speech or whatever in a community it can be readily weaponized by bad actors to disrupt the community.