The beast is very important, it represents the way the boys try to convince themselves that there is no evil in them by making someone else seen to be the cause of evil.
On the very first day on the island, when the little boy with a mulberry-coloured birth mark on his face told everyone of “beastie” which he apparently saw. At the time the older boys thought it was all in the little boys head, his imagination, but its clear that the younger kids were worried by the little boys word.