I'm trying to write a horror short story about a shape-shifter, but I have trouble keeping it from sounding like a science fiction story.
The story starts out with a creepy tone, with a person close to the main character dying/disappearing, but I either have to have the main character run indefinitely, or explain why the shape-shifter is chasing them. Once I explain it, it doesn't seem scary no matter what I do.
Once I flesh out the villain character (think something like It Follows), it's less scary. Adding a character and back-story seems to take away some of the scary unknowns about it.
I noticed several horror and sci-fi books/movies share similar themes, but they're treated differently.
For example, shape-shifting:
It has a magically shape-shifting clown that becomes your worst fear and kills you. Horror.
The Thing has an alien parasite that kills you, then becomes you and kills your friends. Science Fiction and Horror.
Terminator 2: Judgement Day has a shape-shifting robot sent back from the future to track you down and kill you. Science Fiction.
I've been trying to figure out what types of motivations and characters separate It, which is classified as horror, from Terminator 2 and The Thing, which are classified as science fiction. Why am I much more afraid of It than the T1000?
How do I prevent my "horror" story from becoming less scary while still giving the shape-shifting creature a motivation?