By being unnatural and threatening the protagonist with death, the antagonists of horror stories tend to be abominable, meaning they are so evil and irrideemable that they are meant to be hated. Ironically, most fans of horror love the antagonists like Dracula, Freddy, Chucky, and Pinhead as much as their victims, yet this doesn't change the fact that, to the characters in the film, these monsters cannot be redeemed. Even villains who were undeserving victims like Candyman or Jason cannot gain sympathy with their protagonists without the protagonists crossing over into the darkness as well.
To contrast from the abominable attributes of the antagonist, the story usually involves an innocent who is "pure of heart." Sometimes, the innocent is the protagonist, like Nancy Thompson in Friday the 13th. Sometimes, the innocent is protected by the protagonist, like how Tommy is protected by Laurie Strode in Halloween. Often, an innocent becomes a victim of the abominable monster, like Casey Becker in Scream. By contrasting the pure innocent against the abominable villain, horror raises the stakes and make who the audience should root for clear.