There's a strange aspect to slasher movies, one that may have more to do with audiences and even more to do with the state of criticism in general. What I mean is this: while they're basically horror movies of varying degrees of style and shock, they're expected to show their allegiance in terms of sexual politics. Is the heroine/"final girl" virginal, or the opposite of that? Who does the killer target and what are they doing with each other? All this is evaluated in terms of how feminist and sex-positive (a verbal🚩if ever there was one) they are.
That's one of the things that makes the 1974 chiller Black Christmas refreshing. The killer, who calls himself "Billy", isn't enforcing sexual conservatism, nor could you use him to subvert the patriarchy. He's just nuts. Part of this, you could say, is because the subgenre was still in its cradle at the time, four years before Halloween and five before Friday the 13th. But really it goes beyond that. The heroes of the movie―both the sorority girls and the police―make assumptions about the case that lead them so far astray, they still haven't figured things out at the end. And while the red herring for the killings is definitely an asshole, he's not bad in the way he's assumed to be. So in part it's about not seeing what's in front of your face because of what you believe should be there.
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Another note about the movie. One of the sorority sisters is played by a pre-SCTV Andrea Martin. The movie's Wikipedia page claims that Gilda Radner was offered this same role but turned it down due to her commitment to Saturday Night Live. This seems very dubious to me, considering the movie came out before SNL―how you say?―existed. Radner wouldn't have had any real time commitments until the summer of '75, when it would be time to start promoting it. My conclusion here is that urban legends come easy and die hard.