The first reference to masks comes at the end of Chapter Four of Lord of the Flies, Jack describes to the other boys how he was able to kill the pig that is being roasted:
I painted my face--I stole up.
Jack has crawled on his hands and knees to be close enough to slay the pig. In Chapter Five, Jack's cruelty and insensitivity has increased since he has donned the mask for hunting. Then, in Chapter Eight, Jack and the hunters steal the fire from Piggy, Ralph, and the others. Anarchy takes hold of the island, as Jack appears before the boys, saying,
"Listen all of you. Me and my hunters, we're living along the beach by a flat rock. We hunt and feast and have fun....Tonight we're having a feast. We've killed a pig and we've got meat...."
He paused and looked round. He was safe from shame or self-consciousness behind the mask of his paint and could look at each of them in turn.
After Jack and two savages depart, Ralph and Piggy try to reason with the other boys. Ralph tells the boys,
The fire's the most important thing. Without the fire we can't be rescued. I'd like to put on war-paint and be a savage. But we must keep the fire burning.
However, rebellion starts as Bill suggests that the boys go up on the mountain and explain that they need more people to keep the fire going. Besides, Bill suggests, being savages might be "jolly good fun."
The hiding of their visages with paint indicates the boys' descent into savagery. With no person is this descent more evident than with Roger, who now can indulge his sadistic nature. He drives his spear into the end of the sow; he loves the hunt as it affords him the opportunity to inflict pain. And, with the paint as a disguise, Jack becomes completely uninhibited. Of course, when this savagery is unleashed, the intuitive Simon becomes its victim as does Piggy and almost Ralph.