Simon is the only one that recognized that the beast wasn't some physical being prowling around the island; the beast was inside of each of them. It is that "beast" inside that caused the ruination of their island society and the breaking up of any civilized behavior. Obviously, that beast was stronger inside characters like Jack and Roger than it was in the other boys, but Golding suggests that the beast does live inside all of us.

Of course, in a great use of irony, as Simon is attempting to tell the other boys what he knows about this beast, he is killed by a tribe that thinks he is the beast.