Chapter nine of Lord of the Flies is critical to understanding William Golding's theme about the consequences of unrestrained human nature. The entire chapter is set in the backdrop of a massive storm, complete with sweltering heat, black clouds, thunder and lightning, and pouring rain, though it all builds to a climax at the end of the chapter. We have just dramatically met the Lord of the Flies in the previous chapter, and Simon is still recovering from his vision of the devil in the form of a severed pig's head.
Ralph and Piggy have joined Jack's tribe at the top of the mountain for a feast after Jack and his hunters have killed and roasted a pig. Because he has provided meat, Jack assumes the rest of the boys will want to join his tribe, but there is an argument, as always, and there is a sense that there is going to be trouble. The thunder begins to crash and, as they have done several times before, the boys end up in a circle chanting in what they call their "dance."
Amid the chanting and the dancing and the storm, a tiny being crawls out of the forest with a message, a word of knowledge for the other boys on the island. Simon is crawling on his hands and knees, still in a weakened condition from his episode with the pig's head. Simon tries to protect himself from the spears as he says something about a body on a hill. What he is trying to tell them, of course, is that he was right all along--the beast is human, it is them.
In the following passage, the boys in the circle become the beast killing one of their own, something they see as the beast.
The sticks fell and the mouth of the new circle crunched and screamed. The beast was on its knees in the center, its arms folded over its face. It was crying out against the abominable noise something about a body on the hill. The beast struggled forward, broke the ring and fell over the steep edge of the rock to the sand by the water. At once the crowd surged after it, poured down the rock, leapt on to the beast, screamed, struck, bit, tore. There were no words, and no movements but the tearing of teeth and claws.
Simon tries to tell them the truth, that they (all of the boys, all humans) are the beast, and they prove him right as they crunch and scream, surge and leap, tearing with teeth and claws as they kill one of their own. This is the crux of Golding's belief about human nature: man will devolve into evil savagery if there are no civilizing restraints to keep human nature in check.
After the boys kill the beast, Simon, and throw his body over the cliff and into the ocean below. The final vision we have of him is after the storm, and his body is carried away by the glowing creatures of the sea.