Lord of the Flies speaks to Simon at the end of Chapter 8. Here he tells Simon that he is "the reason why it's no go? Why things are what they are?" He elaborates with "Fancy thinking the Beast was something you could hunt and kill!" He then tells Simon that Simon knew all along who the beast was.
When Simon stood up at the assembly that Ralph called in Chapter 5, Simon declares that the beast is "us." The beast is inside. Evil is inside. The true threat to the boys' survival is not from without, but from within. Golding shows us the power struggles, the differing priorities, the increasing violence on the island to portray the source of evil--it is the boys themselves.
Golding wrote the novel in an attempt "to trace the defects of society back to the defects of human nature." It is human nature that is flawed, that is evil. The Lord of the Flies is a symbol of that evil and because Simon refuses to "play" (engage in savagery), he becomes a victim of it.
In chapter six, Simon's theory about the beast is that there really is a beast, but itsnot a physical beast out in the world. Instead, it is evil that is inside of them all-inside of all people. But the problem is that he cannot get this across to the group. When he tries to explain it to them, he asks them to think of what the dirtiest thing in the world is. When he does that, Jack says something vulgar-we are not told exactly what, but it seems fairly obvious that he is referring to feces. Once Jack says this, all the kids laugh and Simon has no chance to get his point across.