In many ways Simon has been depicted as a Christ figure in the book. He has a clear vision of what is happening, in some instances almost prophetic. He manages to exist in between both groups and also clearly cares for and takes care of the littluns. As time goes on, he begins to see more and more clearly what the "beast" actually is.
When he goes off into the woods and falls into a fit, his vision becomes clear and he speaks to the "Lord of the Flies." After his conversation, he knows that the beast is inside of them and he needs to go and tell the boys the truth.
In my version of the book, Simon emerges from the jungle on page 219 and falls into the middle of the dancing and chanting and the boys fall on him and beat and stab him to death.