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. In fact, a beast does roam the island, but not in the form the boys imagine.
Part of Golding's intent was to demonstrate that the evil is not limited to specific groups of people or situations. On the island, the beast is manifest in the deadly tribal dances, war paint, and the manhunt; in the outside world, that same lust for power and control plays out as a nuclear war.
Prior to the war, some of the boys, such as the perpetually victimized Piggy, experienced the brutality of others on the playground, an environment often idealized as the joyous site of a carefree childhood. Within civilized society the beast expresses itself in various ways: through acceptable venues such as the military; in unacceptable forms such as madness or criminality, which carries punitive repercussions; or concealed in the maneuvers of politics and other nonviolent power plays.
In Lord of the Flies Golding illustrates that evil is present in everyone and everywhere; humankind's work lies not in the impossible mission of eliminating it but in the struggle to keep evil from becoming the dominant force in our lives.