In Lord of the Flies, the beast begins as a product of the boys’ imaginations. The smaller boys are afraid of things they see at night; rather than be blindly afraid of The Great Unknown, they give their fear a name and a shape in their minds. You can’t defeat a "nothing," but you can hunt and kill a "something."
The next evolution in the myth of the beast is the dead parachuting man. It’s no coincidence that the boys catch a glimpse of a dark, UNKNOWN object and immediately call it the beast; we wouldn’t be surprised if they were relieved to finally have seen the thing. It’s kind of like how the masters of horror films don’t actually show you the horror, because what you can imagine is worse than anything you could see. Of course, it’s interesting that Golding chooses to make this manifestation of the boys' fear a man -- and not just a man, but a solider coming in from the war. Not only that, but the parachuting man flies in, in response to Piggy’s request for a “sign” from the adult world. It’s ironic that the best the adults can come up with is a man dead of their own violence, and it hints at the allegory and the end of the novel.
This is the point where we start getting some real insight into the beast, via Piggy, who says the beast is just fear, and via Simon, who insists that the beast is “only us.” This is an interesting comment, since the beast is literally “only us:” it’s a person that fell from the sky. In fact, when the twins list off the horrible attributes of the creature they saw, they reveal that it has both “teeth” and “eyes.” Yes, that’s right, most people have teeth and eyes. So Simon is correct in more ways than just one. Even more interesting yet is the moment when Ralph and Jack discover the dead man and think of it as a “giant ape.” What have the boys started to prove except that man is nothing more than a giant ape himself?
But while the beast is in fact literally a man, that’s not what Simon means when he says that it is “only us.” He’s talking about the beast being the darkness that is inside each and every one of us. If this is true, then, as the Lord of the Flies later suggests, it is absurd to think that the beast is something “you could hunt and kill.” If it’s inside all of us, not only can’t we hunt it, but we can never see it, never give it form, and never defeat it.
When Simon has his meditation-scene with the pig’s head, the Lord of the Flies says to him, “I’m the beast.” This makes his other words literally true; you can’t hunt and kill the beast, because they’ve already hunted and killed the pig and it’s still talking to you. Even later, when Ralph smashes the skull, he only widens its smile, “now six feet across” as it lies “grinning at the sky.” This thing just won’t die, and it torments Ralph so much because it “knows all the answers and won’t tell.”
Now to Ralph, that’s a rather silent devilish pig’s head, given that four chapters earlier it was talking with Simon. It seems that the Lord of the Flies gave over its knowledge to Simon, but only to Simon. In his death, then, Simon took that wisdom with him. What wisdom are we talking about? Simon already knew, it seems, that the beast was simply the darkness of man’s heart, but the talking pig’s head actually confirms it, telling him “I’m part of you […] close, close close.”