When Piggy tells the assembly that there is nothing to fear but people, he is foreshadowing the violence on the island amongst the boys and his own death.
Though he is not respected by most of the boys, Piggy calls an assembly to remind everyone of the rules. They have to remember to use the rocks as a bathroom, for example. They have to be careful of the fire, and they have to keep it going so they can be rescued.
He scoffs at the idea that the forest is dangerous.
Of course there isn’t nothing to be afraid of in the forest. Why—I been there myself! (ch 5)
Piggy tells them that life is “scientific,” and the war will soon be over. There is no fear unless they “get frightened of people” (ch 5).
Some of the children laugh it off as a nightmare, and others are frightened. On some level, the boys realize that the beast they are afraid of does not live within the forest, but within themselves. It is their deepest, darkest desire. It is the violence and mayhem to which they can succumb. When they do, it is as if Piggy saw his own death.