The beast in the Lord of the Flies is a central theme. Almost from the beginning of the novel the boys know that not all is well on the island. In chapter two, the Littluns claimed to have seen the beast. To them it was a serpentine creature.
As the novel progresses, more and more of the boys start believing in the beast. Jack believes it is a hunter; Ralph believes as well, even if he does not know what it is. Simon is the most insightful. He begins to come to the conclusion that there is a beast and the beast is within. For him, the beast is our desire for evil.
From this insight, we can say that Golding is making a profound point. Human nature is corrupt. Even boys in a "pristine" island will come to ruin, because they are people. Without laws and society to reinforce "good" and "proper" behavior, "evil" will hold sway, which happens in the book. Order and civility are gone by the end of the book.
The head of the "beast" make this point eloquently:
“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?”
The importance of the beast develops as the boys become more and more entrenched in their new life. Initially, the boys fear the beast. This parallels the fear they are experiencing. Their relationship with the beast develops as the boys become more and more savage in their behavior. By the end, the belief in the beast becomes stronger and they begin to bring it sacrifices and treat it as a godlike figure. The more savage they become, the more the beast exists and becomes prevalent.
The beast acts as a symbol for their savagery.