If you've seen enough slasher films, you know it's a standard for the killer to not only be revealed at the end in dramatic fashion, but also to have an elaborate backstory that provides method for their madness. Frequently this is conveyed by the killer themselves in a climatic rant (the "Scream" films in particular have deliberately taken this to an art form as expected from killers who get all their motivation from the genre). In the end, it's expected that we will know everything about the killer and why they do what they do. It's almost reassuring, as if letting the audience know, "See, it was just some sad lunatic and now he's put down. Everything's fine." Even Michael Myers, who was originally designed to be an unknowable vessel of pure evil, fell victim to this and in spectacularly stupid fashion (seriously, the Cult of Thorns bullshit was clearly the product of a screenwriter with too much time and too many drugs on hand).

And this is where "Black Christmas", pre-dating even "Halloween", goes gloriously in the opposite direction regarding its killer Billy. Because we find out nothing about this guy. Sure, we learn his name is Billy and his deranged ramblings hint at a horrific backstory involving poor Agnes, but that's it. We don't even see his face (except in shadow), just those horrifying insane eyes of his. There's no climatic showdown, no speech where he explains his motivations, nothing. There is no release given the audience in watching him unmasked and taken down. He's still an unknowable, terrifying enigma and he's still out there and ready to strike when you least expect it (and quite possibly murder the sleeping Final Girl).

That is a big reason why "Black Christmas" remains such a terrifying film. There is no release in explaining Billy. He's the unknown evil still out there and ready to kill again. Just like all too many real-life killers.