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.