you might do well to look into several of the sub genres. Found Footage is prominent, as are the occult, apocalyptic, and suspense-thrillers. you also may find film noir and neo-noir in the collections of horror aficionados alongside such staples as tragic horror, b-movie horror, the macabre(widely regarded as it's own genre), psychic horror, giallo, extra-terrestrial horror, disaster horror, fairy tale horror, lovecraftian horror, and grotesqueries. Try toying with your ideas of horror, as you will find fantastique, jiangshi fiction, weird fiction and it's child the new weird. delving deeper into the depths you will find gothic and it's cousins southern gothic, suburban gothic, tasmanian gothic, and urban/suburban gothic. further still you might find grind house, holocaust horror, early german expressionist films (the cabinet of doctor caligari), mutant horror, documentary horror, and serial killer stories. Down the rabbit hole you will find bizarro horror, torture flicks, historical horror, horror of personality, korean and japanese horror (see Ringu, the host, Audition, and the grudge most of the former and latter of which have been americanized for non-bilinguals), autobiographical horror, reincarnation horror, the Penny-dreadfuls, erotic horror (not suggested), and what some call Hagsploitation (Older woman in supernatural or terror-inducing peril) and blaxploitation (see the b-movie franchise blackula for examples). Crossgenre horror artists have produced such sub-genres as virtual reality horror, time and space travel horror, horror rooted in psychic powers, infection horror, action horror, horror of comedy, Drug Horror, and of course, Supernatural Romance (see examples such as let me in, or it's counterpart let the right one in, interview with the vampire)
Oh, and don't forget Romance (no, not romance, but Romance) It's a very old genre which provided horror, and gothic literature with a place to grab on and fester and boil into it's current melting pot of all that it has become.
Lastly, and I only mention this for people who watch every aspect of the genre, is a sub genre which critics and scholars alike have dubbed torture porn (nothing like how it sounds. for examples, look for such films as A serbian film, the Saw franchise, Hostel (parts 1-?), and the human centipede.
Furthermore, I think you will find certain philosophical films, such as Begotten(1990) to be of great interest in your search for what constitutes horror.