What message is William Golding trying to convey in Lord of the Flies? How does he carry it out throughout the book?

In Lord of the Flies, William Golding reveals the frightening absurdity of barbaric human behavior that results from a lack of order and civilized thinking. Left alone on the island, the boys are certain to self-destruct. Through a detailed characterization of several young boys whose plane crashed on the island, Golding conveys humanity's tendency toward chaos. 

At first, the boys begin to build shelter and find food for survival. Ralph is the leader who realizes that he must keep a fire going to alert a passing ship of the boys’ need for rescue. Eventually, Ralph begins to feel that his efforts are futile because he cannot maintain order without the support of the arrogant choir leader, Jack. Ultimately, Jack becomes blood thirsty and strangely focused on hunting pigs. He loses all respect for human kind as he leads one group of the boys to challenge and attack the order or structure of civilization.

Golding contrasts Ralph and Jack to convey the message that dangerous chaos can result from a lack of authority. 

Throughout the story, Golding reveals Jack’s dangerous character through Jack’s evil obsessions and actions, which parallel the war being fought by adults in the larger world.

While isolated from civilization, the boys’ behavior leads to a dreary outcome. Golding examines mankind’s basic instinctive behavior to destroy itself. This is manifested in the actions of Jack and his hunters as they forget civility and plunge into savage, barbaric actions. No doubt, Golding, who has experienced the destructive behavior of mankind as he served in World War II, has come to the conclusion that society has a bleak future.

Through the contrasting of characters, Golding conveys that there is a clash between those who respect civilized human actions and those who lose any semblance of civilized, orderly, safe behavior. The novel is an examination of good versus evil. The various characters display opposite behaviors:

Ralph, Piggy, Simon, Sam and Eric see the need for order and civilization, while Jack and his hunters become obsessed with the ideas of finding meat and protecting the littluns from the beast.

Ultimately, the boys are divided and abandon safety and a desire to be rescued. Golding is conveying the message that mankind will destroy itself if left to the innate, barbaric savagery that is directly below the surface of human thinking. Human nature is not to be trusted. The atomic warfare going on all around the boys at this time proves that when mankind is left alone to carry out his destructive innate behaviors, civilization does not stand a chance. Hopefully, Golding’s message, a powerful truth, was shocking enough to cause mankind to sit up and pay attention to the destructive savagery that could cause mankind to cease to exist in any civilized manner. Golding proves this when Jack and Roger become murderers of their own choir school brothers. Things become so bleak on the island that even Ralph cries in the face of rescue. He realizes the naval officer represents the ideas of destruction through uncivilized atomic warfare and Ralph is not sure he wants to be rescued from the savagery that exists in mankind.