Horror, science fiction, fantasy - these genres are often grouped together under the mantle of "speculative fiction." In speculative fiction, the author indulges in speculation - the contemplation of things not known, according to one definition.
As writers, we are often told by creative writing instructors (there's a special place in hell for them) to "Write What You Know." It's one of those golden rules that you have to learn how to break. It is important to write what you know. However, you shouldn't limit your writing to what you currently know.
I get lots of backyard stories - that is, stories written in the author's tiny backyard of knowledge and experience. This isn't always a bad thing. There are stories to be told here, but for them to evolve beyond the backyard and become speculative fiction, they must speculate beyond what you currently know. You must research, expand your horizons of knowledge, or be an accomplished bullshitter able to make up alien landscapes of such imagination and depth and scope as to become vividly real to the reader. Your stories must be written in such a way as to make the reader care about what's going on in your backyard. Are there interesting smells coming from your barbecue? Does it sound like people are having fun there? Is your garden secret or special enough to wow the neighbors?
In the old guidelines for Dungeon magazine, they listed a number of plots to avoid. To paraphrase one of them, a sane wizard with real motivations, foibles, strengths and weaknesses is much more interesting than an insane wizard bent on destroying all life as we know it because he's, you know, nuts. The same holds true for serial killers. Completely crazy Ed Gein type serial killers lounging on couches upholstered with mama's skin maybe be horrible in a freak show carnival way, but they don't make for good stories unless there is something more to them than their insane compulsion to decorate lamp shades with women's fingernails. It's impossible to truly get inside the head of a serial killer, so you must speculate about what really motivates him. You must make him or her unique, not only to you, but to the reader. The editor is the first (and most difficult) reader you have to convince.
Because trust me, my slush pile is bursting with serial killers. They all eat their victims, too, or collect their eyeballs, or whatever. A pinky-toe collecting serial killer isn't likely to excite us, even though we haven't read any pinky-toe collectors. Half the population thinks the scariest thing in the world is the mad-I-tell-you serial killer. But this is hell. We play bacci with the Ted Bundy's skull. Leonard Lake shines our hooves. Chances are your story's serial killer isn't going to make it out of the Lake of Fire.
H.P. Lovecraft wrote what he knew - rural New England - but he placed into it horrors of his own imagination. And he didn't have to dig deep to find them. As Holmes says in "The Adventure of the Copper Beeches,"
It is my belief, Watson, founded upon my experience, that the lowest and vilest alleys in London do not present a more dreadful record of sin than does the smiling and beautiful countryside... But look at these lonely houses, each in its own fields, filled for the most part with poor ignorant folk who know little of the law. Think of the deeds of hellish cruelty, the hidden wickedness which may go on, year in, year out, in such places, and none the wiser.
Therein*, dear writer, you too can find your own personal glimpse of hell with which to populate our simple postcards. Find a unique place that you know very well, a place where horror can take hold, and then fill it with your nightmare. Because the best horror hides amid the ordinary. That's what makes it believable, and to be an effective horror story it must, above all else, be believable.
*No, this is not a specific call for farmhouse horror. Although I would appreciate seeing a few more stories set outside the generic urban/suburban American landscape.