He was a total wack job. slicing off skin off of his head just so he could be accepted by them.
Why would they even let him in even if Boone died from getting shot up and came back to life?
Extremely late response but the novella explains it a lot better. In the movie he does come off as a wack job but he is not human.
In the book (and partially in the movie), he says he has a real face underneath his human one and he’s spent his whole life just knowing he was an outsider. In the novella, he completely removes his face to reveal that he is some sort of scaly demon/monster. In the film, you can see he has some sort of scaly hide where his scalp should be. He also has an incredibly high pain tolerance, likely after he was welcomed in as Nightbreed and went through the whole ceremony—which might have erased any trace of humanity he had.
After his death in the novella and director’s cut—he comes back in the comics (not decapitated) and Boone is surprised and it confirms he is not a human but some sort of creature. The writers of the original BOOM comics intended on exploring his seeming inability to die or suffer mortal blows in an unpublished issue that was to be called “Bring Me the Head of Narcisse” but they got gutted and had to rush through three major story arcs in the last issue—ignoring all the character building they had done.
Nonetheless, it is safe to say Narcisse is not human. What is he and why? Who knows—at least he looks cool lol