That’s where an element of mystery can turn a good horror tale upside down.
This subgenre can take the form of a traditional slasher. Kills that involve first-person points of views allows us to see the vantage of the killer, and the mystery of who it is can add to the tension throughout the story. We see at least elements of this in the first Friday the 13th and Sleepaway Camp.
Other times horror mystery can take the form of crime tales. Se7en is a great example, but there are many others. Even Sir Arthur Conan Doyle has an example of what many would consider the first of the subgenre with “The Murders in the Rue Morgue.” That story is often credited as possibly the very first work of detective fiction, but it also could be considered an example of horror mystery.