There are advantages and disadvantages to writing in any genre, and horror is no exception. While it is a relatively small market when compared to romance, police procedurals, or political thrillers, the possibilities of horror extend much further because of its presence in every genre – including children’s lit and mainstream fiction. In fact, it’s so prevalent in romance, that it has its own thriving sub-genre, “paranormal romance.”
One of the advantages of horror is that while it can be a genre in its own right – think vampires, ghosts, and other monstrous beasties – it is everywhere. The Silence of the Lambs is considered mainstream, yet it is a horror novel throughout, passing the genre tag perhaps because it is not only well written, but Hannibal Lecter is a human monster. (His extreme senses are much like those of detective Sherlock Holmes’ in that they can seem supernatural. But in Hannibal’s case, they are used to foment crimes, not solve them. That’s the only difference.)
Speaking of the Great Detective, several of Sir Arthur Conan Doyle’s tales – such as The Hound of the Baskervilles – appear supernatural although ultimately, the hound and other mysteries are explained away. If the hound had remained a possibly supernatural creature, the same novella would have been labeled horror in a heartbeat.
Horror is subjective. Aliens is science fiction but is really just a haunted house story set in a spaceship and the bogeyman – er bogeywoman – is from another world instead of the depths of our closets.