Why doesn’t Simon believe in the beast?

In chapter 5, it has become apparent that order is vanishing among the boys on the island.  A meeting is held to, among other things, determine what is the problem.  Piggy suggests that fear among the boys, especially the littluns, is part of the problem.  The boys talk about whether or not there is a beast on the island.  Simon is asked his opinion and he tentatively says that he isn't sure; that maybe it's only them. He means that he doesn't think there is a beast with claws and fangs; something living and breathing outside of them. He thinks that the beast is the evil inside of each of them. Simon is the mystical thinker among the boys and he has been quietly watching them and their interactions.  He seems to understand things on a higher level than the other boys.  He, like the other bigger boys, hasn't seen a physical beast and common sense makes him doubt the existence of such a thing. He has seen, however, the way the boys act and the personalities of the boys, and he understands that there is an inner evilness at work among the boys.  When he sees the parachutist in chapter 9 after his chapter 8 conversation with the Lord of the Flies where he's come to fully understand that he was right about the source of evil on the island, Simon frees the parachutist from his strings and runs to tell the others of his discovery.  They kill him in their frenzy before he can tell them.