Simon begins to hallucinate and wonders into the woods. one of the things that he thinks he sees is the Lord of The Flies talking to him.
Simon wonders around and ends up climbing the mountain, only to find out that "the beast" is just the body of a dead parachutist
Simon is excited about this news that he has discovered and wants to tell the other boys immediately, so he goes back to where they are.
Simon comes back to tell the boys the truth about "the beast" when he sees the boys are all dancing around pretending to be a pig, and imitating killing it... they have all gone a bit insane. when they see a figure (Simon) coming out of the trees, their instinct is to kill it.
The boys start attacking Simon as he approaches them with the news. Simon falls and dies and his corpse gets washed away at sea.