The beast represents the primal, instinctual side of human nature that is often repressed in civilized society. For example, when Ralph and Jack are fighting over who will be chief, Jack says that he’s going to hunt for food and kill pigs and make fire for the tribe. Ralph tells him that he can’t do that because it’s against their rules, but Jack insists on doing what he wants. When he goes hunting, he finds a sow with piglets and kills her by smashing her head with a rock. When he returns to the beach with her piglets, the other boys are impressed by his bravery and skill at hunting; they let him become chief after all.
The beast also represents dark impulses that lurk within all of us—especially those who have been isolated from society for too long—and can emerge under the right (or wrong) circumstances. For example, when Simon sees Piggy being attacked by Roger and gets angry enough to start throwing rocks at Roger himself, it shows how easily someone can give in to their darkest instincts if they feel threatened enough by something or someone else’s actions.
The beast also represents fear of the unknown or power of imagination.
