Simon's conversation with the beast is imagined and comes from within himself, which is where, of course, the beast is in everyone. As Simon imagines a conversation with the pig's head, the Lord of the Flies, it tells him, “Fancy thinking the Beast was something you could hunt and kill! . . . You knew, didn’t you? I’m part of you? When the beast tells Simon he is part of him, Simon understands. This leads the beast to reveal that the reason they haven't been able to do anything is because the beast is part of them. Their characters are too flawed with evil. Nevertheless his conversation with the beast within himself foreshadows events that will occur later. For instance, at the end of the chapter, Simon imagines that he falls into the mouth of the beast as he slips into his seizure. This foreshadows the violence that will later consume him and the others as they slip into beast-like savagery.
In a commentary that Golding gave about his reason for writing his story about a group of boys instead of a group of girls, or even a mixture of boys and girls, he stated that he wanted to write a story about what would happen if a group was stranded on an island--it would be a boiled down version of society, and Golding claimed that he could do that with boys, not girls.
Where does this commentary fit in with the question? In Simon's vision, he sees the other part of humanity, the evil side, the side that questions everything and reveals only a pessimistic view of the world. Golding claims in this novel that evil can win in the end if we allow it; if we do not listen to reason and compassion, represented by Piggy and Ralph, respectively, then evil, represented by Jack, will overcome the rest who are not strong enough to resist it.