The pig's head or "Lord of the Flies" warned Simon when he says "I am part of you." In other words, the evil that the Lord of the Flies represents is inside of everyone. That why we can't eliminate it. Simon's death is an example of that assertion. Simon runs down the mountain thinking that once he explains that the beast is simply a parachutist, all will be fine. He underestimates the evil inside of the boys and ignores their behavior which shows they are in the middle of a ritualistic dance where the evil inside of them is controlling their behavior. The mass reaction to Simon is an indication that the evil is part of inside all of the boys and Simon's news will not change that. In fact, he is never given even the chance to share his news because he is killed as the boys turn into an evil pack bent on his destruction.