In chapters 5-6 of "Lord of the Flies," what does Simon mean when he says the "beast" may be the boys themselves?

During the assembly held at night to put things back in order, Ralph opens up a discussion about "the beast," hoping to settle the matter once and for all so that they can "start again," and "be careful," and "be happy." However, the discussion has the opposite effect as the possibility of a "beast from water" begins to seem plausible. Simon, who is more sensitive and discerning than the other boys, but who has difficulty expressing himself in front of a group, rises to speak. He starts out by saying, "Maybe there is a beast," and the other boys interrupt him. He finishes his thought awkwardly, clarifying, "maybe it's only us." He is hoping to convey to the boys "mankind's essential illness," the fact that if the boys are in danger, it is from each other and from their own moral failures. His insights are spurned by the others.

In the next chapter as the boys are searching for the beast that Samneric have seen, Simon remains in doubt about its existence as a physical creature. Even when he tries to imagine what type of creature it could be, "there rose before his inward sight the picture of a human at once heroic and sick." Whether he has in mind a political figure from his life back home, or Jack, whom he has acknowledged would be a poor alternative to Ralph as chief, he understands that humans are capable of producing far worse damage to society than beasts can. That is why he mumbles to Ralph, "I don't believe in the beast." He believes the real threat to the boys is the heart of evil that lies within each of them.