Most of the boys have got it into their heads that there's a fearsome beast lurking around every corner, stalking the island in search of its next meal. There isn't, really; it's all just a figment of their boyish, overactive imaginations. Because the boys are young, naive, foolish, and frightened, the beast myth proves a difficult one to let go of, not least because Jack cynically encourages belief in the beast's existence as a means of consolidating his dictatorial power.
So long as the boys are scared, so long as they think they really are in danger from the beast, they will look to Jack to offer them protection. If they only stopped to think for a moment, however, they'd realize that the beast is but a symbol of the darkness and evil that lurk deep within the human soul.