An article recommending horror films for Halloween by Brad Balfour in Times Square Chronicles, 31 Oct 2020:
https://t2conline.com/celebrate-halloween-with-a-night-for-horror-masked-by-cinema/A list for adults, and things like The Shining, American Werewolf, Alien etc seem fine. But The Devils?
This gets to people regularly classifying The Devils as a horror film. The Cambridge English Dictionary defines a horror film as "a film in which very frightening or unnatural things happen". The Devils fits this category (frightening things) but so does The Killing Fields, Zero Dark Thirty, The Irishman etc.
Surely the essence of a horror films (my definition) is that it is "a film in which the primary entertainment is from frightening or unnatural things happening". So the horror is the main intention and the main entertainment. What audiences want to see.
This would include The Exorcist, Jaws, Ring, Blair Witch Project. The films may have a sub-plot- Dawn of the Dead and consumerism- but people go expecting to be frightened.
There are hybrid films, horror-comedy (Lair of the White Worm, Shaun of te Dead) and sci-fi horror (Alien, The Thing).
And The Devils. What are the main themes? Partly the struggle between the Church and the State (look at the pre-credit sequence), partly about a sinner (a priest with a mistress) who is also a saint (he sacrifices himself for the city). The horror (the torture of Oliver Reed) is to back up these themes, and no-one goes to see the film mainly for the horror.
That was my rant.