Back in the day it was considered horror. I used to love all of the 'American-International' movies. I called them the 'Drive-In' movies. Here is an article about the studio and the movies they made.

https://en.wikipedia.org/wiki/American_International_Pictures

@mechajutaro said:

more of a period drama, i would say.

The perils of becoming enmeshed with labels and categorization. Wouldn't be a stretch to relabel George Romero's Martin a teen tragedy or the first Insidious a dark fantasy. Hell, one could deem The Silence of the Lambs either a crime drama or police procedural. Such hand wringing quickly degenerates into homing in a grain of sand to the exclusion of the shore itself.

Insidious is Fantasy. as is a slew of Horror. i've not counted but i might expect even the majority. i'm not familiar with Martin.

i wasn't trying to claim that genre designations supercede one another. my point is that the film i am referring to has very little to distinguish it as Horror at all.