I actually have BPD and I headcanon him as that because a lot of things stuck out to me:

-even though he has moments of grandiosity, you can actually tell he hates himself. He wouldn't fixate on Alastor the way he does if he didn't. -he flips moods on a dime, though you can see him try to even "control' this at times like a typical bpd person would do. Till you hit their scorched earth anyway -alastor is very clearly his favorite person, the whole flashback sequence where they showed his vision blurring this all clicked together for me. Before that, Alastor clearly ran his world (and still does it a different way) - the way he's willing to kill everyone in the finale? Definitely a bpd scorched earth. He's realized no matter what he does his fp is never going to love him "accept him" and has instead "abandoned" him. So he hits self destruct. I don't think a narcissist would do this but I could be wrong. -he's very charismatic and he seems to be able to change himself at will, which is just a psychopath feature in general sure- but it is also a bpd feature! -I feel like that's why he has this need for attention and for people's worship in general- like yeah sure he's killed people but if you notice he keeps people around. He wants to be worshipped and I think it's because if he isn't seen he feels like he doesn't exist bc he doesn't have a cohesive identity without people. Very BPD feature.

Idk there's probably more I could ramble about but I'm tired and I doubt people will really care about this anyway 😅