The Lord of the Flies lets Simon know that there is no beast except for the beast within men. The head says:
"Fancy thinking the Beast was something you could hunt and kill!" said the head. For a moment or two the forest and all the other dimly appreciated places echoed with the parody of laughter. "You knew, didn't you? I'm part of you? Close, close close! I'm the reason why it's no go? Why things are the way they are?"
This illustrates the theme that humans are inherently evil. Simon knows this from earlier on in the novel. He seems to be more spiritual in his behavior than the other characters. He goes off into the woods and into his hiding places. He chooses to remove himself from their little society. He says all along "maybe it is just us" rather than their wild stories about air beasts or water beasts.
All along, Simon knew that there was no physical beast that would destroy them all. He recognized that it was their own fear that would destroy them. It is the boys' fear and desire to destroy the beast that kills Simon.