Golding uses the boys' fear of a mythical beast to illustrate their assumption that evil arises from external forces rather than from themselves. This fearsome beast initially takes form in their imaginations as a snake-type animal that disguises itself as jungle vines; later, they consider the possibility of a creature that rises from the sea or the more nebulous entity of a ghost. When they spot the dead paratrooper who has landed on the mountain, the boys feel sure that they have proof of a beast's existence. In fact a beast does roam the island, but not in the form the boys imagine.
Golding wanted to illustrate in this novel the dark side of human nature and make the point that each member of humankind has this dark side. The boys conceptualize the source of all their worst impulses as a beast, some sort of actual animal or possibly supernatural creature inhabiting the island. Yet all along the boys take on the persona of the beast when they act on their animal impulses. There is no external beast.
