What is the significance of Simon's death in Lord of the Flies by William Golding?

The brutal murder of Simon marks an important turning point in the novel in terms of plot, characterization, and theme. Just prior to this scene, Jack has called for a vote of no confidence in Ralph, but the vote doesn't go Jack's way, so he says he's not going to play anymore and leaves the group. It doesn't take long before he has drawn many boys after him who want to hunt and honor him as chief. When he successfully kills a pig and hosts a feast on the beach, every boy except Piggy, Ralph, and Simon is lured over to Jack's camp. Eventually even Ralph and Piggy succumb to the temptation for meat. Thus the power structure has already begin to tilt away from Ralph, who represents civilization, and toward Jack, who represents violence. When Simon is murdered, it creates a further division in the two camps. Ralph, Piggy, and Samneric seem to understand what has occurred, and although they deny it outwardly, they at least acknowledge it to themselves. However, Jack uses the event to further solidify his tribe around him, using fear. He reminds the boys how "the beast" crept up on them, and he says they did not kill it, but that it still lurks about the island. 

The way the boys react to the murder reveals a great deal about the types of people they are. As mentioned, Jack uses the murder to bind his followers to him in the most immoral way--totally shrugging off the horror of what they did and using bald-faced lies to manipulate his followers. Ralph seems most impacted by the event. He clearly states to Piggy, "That was murder." From this point on in the story, Ralph finds it harder and harder to think and stay focused on rescue. It's evident that he takes his personal moral failing very seriously. Piggy, on the other hand, is willing to justify their actions, calling it "an accident." Samneric follows Piggy's lead and denies having been a part of the killing, but "memory of the dance that none of them had attended shook all four boys convulsively." Golding makes a distinction between those who feel guilt for the murder and those who brush it off.

Simon's death furthers the theme of the novel. It shows how the boys' society is plunging into depravity. The fact that "innocent" British schoolboys could commit such a heinous act with their bare hands shows clearly the darkness of the human heart. Simon is also a Christ-figure, making his death more poignant. The boys have killed the one person on the island who understood "mankind's essential illness." Looking at the novel from a historical/political perspective, where Jack represents Nazi Germany and Ralph and Piggy represent the Allies, Simon represents the church and/or the philosophical and artistic communities during World War II that were stifled and prevented from having the influence they should have had.