"Fancy thinking the beast was something you could kill." What is the author trying to say with this quote?

This statement is spoken by the "Lord of the Flies," the "pig's head on a stick" that speaks to Simon in his vision in the thicket. While the conversation between the pig's head and Simon can be confusing--and it was written in a way to seem mysterious and ambiguous--it presents one of Golding's strongest themes in the novel. The lesson of the novel is that humans, left on their own even in an idyllic environment, will create a society that quickly degenerates unless they can follow rules and unless they have a moral basis for their lives. Ralph is the character who grasps the importance of rules and order; Simon is the character who grasps the importance of morality. Simon has already suggested that the "beastie" the boys are afraid of is "only us." Here Golding explores that idea further. The head says, "I'm part of you.... the reason why it's no go." The fact that he is called the Lord of the Flies is a reference to Beelzebub, or Satan, the personification of Evil. So Golding here is saying that evil resides within the human soul--"close, close, close!"--and therefore it cannot be hunted and killed as something external. The only way for the boys to really deal with the evil and save their society is to recognize the potential for their own moral failings and to build their society on a moral foundation.