William Golding's novel Lord of the Flies is a symbolic story set on a tropical island; one of the primary symbols in the novel is the beast and the boys' attempts to hunt and kill it.
Early in the novel (chapter two), the little boys, known as the littluns, express their fears about something that has been frightening them. Piggy gets one of the littluns to talk:
“He wants to know what you’re going to do about the snake-thing.”
Ralph laughed, and the other boys laughed with him. The small boy twisted further into himself.
“Tell us about the snake-thing.”
“Now he says it was a beastie.”
“A snake-thing. Ever so big. He saw it.”
While it becomes obvious that the boys are having nightmares over an imaginary beast, Jack and Ralph reassure the others that, if there is a beast, Jack would have seen it. He has not. Despite that, even Jack and Ralph are a little shaken at the thought.
Later, Samneric are startled by a sound they cannot identify (the dead parachutist kind of blowing in the wind) and immediately run like madmen, claiming the beast had claws and fangs and was chasing them. Once again, Jack and Ralph attempt to diffuse the fear, but it is more difficult now.
Simon is the only one who considers that it is they who are the beast--the foul, evil thing--which is allowed to run wild on the island. He just mentions it, quite hesitantly, in a meeting once, but he is quickly shouted down and scorned as foolish. Later, after his conversation with the Lord of the Flies, Simon knows he is right and tries to tell the others; his efforts get him killed--as a beast.
The fact is that the Lord of the Flies and Simon are right; the beast is them:
“Fancy thinking the Beast was something you could hunt and kill!” said the head. For a moment or two the forest and all the other dimly appreciated places echoed with the parody of laughter. “You knew, didn’t you? I’m part of you? Close, close, close! I’m the reason why it’s no go? Why things are what they are?”
If that is true, then in one sense the beast can never be killed because it is something (their evil, unchecked human nature) that only exists inside each of them. In another sense, though, the beast has begun to turn on itself; it has killed two boys (three if we count the mulberry-birthmarked boy) and it is ready to kill another (Ralph). Over time, the beast inside of the savages would have annihilated itself.