The best Christmas horror movie is, of course, It's a Wonderful Life....
.... if you imagine Clarence as a devil sent to torment George for offing himself....
....and if stop the film 10 minutes before the ending.
But Black Christmas is a pretty fair addition to this very specific sub-genre. Margot Kidder and Olivia Hussey as sorority sisters. Keir Dullea as the madman stalking them. Written by Roy Moore...but not that Roy Moore.
If you have seen some of the tropes in this movie before, it's because every movie of the kind that followed? Stole from it.
Enjoy!