Many horror movies are based on the idea of the normal characters (the normals) being eaten by the abnormal ones (the deformed destructive beings, or DDBs). These are phagic stories, tales of being eaten. This is true of zombie movies, and partially true of vampire movies, where what is consumed is only a part of the victim, blood. Berserk biota, natural non-human creatures acting strangely, are always munching on humans: sharks (most famously Jaws ), anacondas, piranhas, rats, resurrected dinosaurs, man-eating plants, and so on. Alien monsters, such as the creature in Alien (1979), also seem to have a taste for humans. So do cannibals, such as in The Texas Chain Saw Massacre (1974).
These movies suggest an insight into the nature of DDBs. Every species that has evolved has to procure food, and most species (aside from most green plants) do it destructively, by devouring some other living thing. Every species that has evolved is deformed with respect to its ancestors: it evolves through deformation. Therefore most species are DDBs, or monsters. Humans are no different. We are monsters to all the animals and plants we eat. A good horror movie would be set in a slaughterhouse, from the point of view of the cow. In fact, that movie was made, with humans standing in for cows: The Texas Chain Saw Massacre.
Horror movies--at least the phagic ones under discussion here--are like recurrent nightmares we have been having since our evolutionary childhood, the prehistoric days when there were many more creatures around that could eat us. Even today, when there is little danger of being eaten, we know that we could be eaten: that we are no less vulnerable to teeth than the animals we ingest. And we know that we survive by eating, giving us cause for both delight and guilt. Unless we ever become indigestible or cease digesting, our phagic nightmares are likely to continue.
Deformed and Destructive Beings: The Purpose of Horror Films