Distinguish between mystery, horror, and suspense:

“In a mystery you might find out that a person was beheaded. This occurs before the narrative begins, so the focus of the story is on solving the crime. If you’re writing a horror story, you’ll show the beheading itself—in all of its gory detail. If you’re writing suspense, the characters in the story will find out that someone is going to be beheaded, and they must find a way to stop it.”

/ Elements of Suspense in Writing: 6 Secrets to Creating and Sustaining Suspense – Writer’s Digest