Like anything else in fiction, it's largely point of view. One kind of horror - the most visceral kind - strikes at the primal POV of the hunted fleeing the relentless pursuit of the predator. The reader in the victim's shoes feels every pulse-pounding moment, every stifled breath in the pitch dark, every bead of sweat. Whether it's group of people being picked off one by one in a dark, haunted forest, as in the "Blair Witch Project" or a woman fighting for her life in the dark, claustrophobic passageways of a futuristic spaceship as in "Alien."The female perspective may sometimes seem more primal, more intense and therefore more powerful in such horror, and it sometimes makes it that much more effective if and when she turns the tables on her adversary in the end. Other times, though, things aren't so primitive or clearly defined. Sometimes a more subtle kind of horror can lie in the mystery of not knowing what's going on. You don't always need a body count to project an effective sense of dread or menace. Sometimes that unseen presence that may or may not be real as you hear a floorboard creaking in the next room is all it takes to get the reader's blood racing.
And sometimes, the final moment can be the most effective when there's a twist in the plot. Usually a twist based on POV. Sometimes, the protagonist we assumed was the victim turns out to be the predator, or the one different from ourselves whom we instinctively fear. It's been said a nightmare's power lies in its ability to rip the world we think we know out from under us. The shattering of preconceptions can be the biggest jolt of all.
Our own point of view is the place we call home, where we feel safe. Shake that foundation, and horror rises from the pit of the unknown.
