"The beast within"

Horror is a genre that has a reputation of being a low, somewhat trashy, titillating genre that appeals to our basest instincts. 
"Its also a wonderful, popular art form through which very complex ideas and creative techniques can manifest themselves—and if you can get past that very cliched view, you realize there’s an embarrassment of riches in the genre.” Malcolm Turney

... it has to have a monster, which has to be threatening in some way. The monster is often otherworldly or violates the laws of nature, as in Alien or Jaws—but some argue that a human character can be a monster, as in Psycho, Silence of the Lambs, and Halloween.

The monster generally is otherworldly or violates the laws of nature and is designed to elicit disgust as an emotion.

This is the most popular theory explaining the genre’s popularity and it argues that an unconscious, repressed part of every human is actually savage; that the veneer of civility is very thin, and beneath that is essentially a monster.

Although we consciously disapprove of what the monster is doing, deep down part of us enjoys seeing the murder and mayhem the monster unleashes—because if we could, we would do that.

• We consciously disapprove of what the monster is doing, but deep down part of us enjoys seeing the mayhem the monster unleashes.
• We enjoy rooting for the protagonists as they battle the monster.
• We might not endorse the monster’s actions, but we may find it captivating. 
• Findind the nature of the monster - its powers, its weaknesses, where it came from  - is a process of discovery that we enjoy.
• The thrill that fear offers, particularly for “sensation seekers" - especially in a context where there’s no real threat to us.
• It contains symbolic representations of real fears.