James Chastek offers a succinct explanation of the horror genre:
One can make any number of horror-movie monsters by removing the limits from some natural desire: Alien is reproduction without regard for anything else; The Blob is pure and unlimited growth (with unlimited consumption as a corollary); The Thing is a sacculina-esque parasite that places no limits on its own desire to survive etc. Removing the limit shows two sides of removing a limit: on the one hand we get a monster and on the other hand we get godlike power. These two aspects are usually reflected in the plot: the protagonists want to kill the unlimited thing as a monster; the antagonists (government officials) marvel and desire the unlimited thing as a god. The marvel of the antagonists is tempered by the fact that they do not simply marvel at the godlike thing but also desire to possess and control it. Such possession itself is a claim to unlimited power, and so is a redoubling of the monster.
Via Brandon Watson .
