Each of the four main characters in Lord of the Flies, by William Golding, represents some aspect of man. Golding wanted to demonstrate his view of what causes evil in this world, so he put a group of orderly school boys on a deserted island without any adult supervision. It is the perfect environment to test what aspects of man most tend to do evil. In this novel, Jack is the embodiment of man's human nature, and he is the primary cause for the demise of civilization on the island.
Soon after he the plane crash, Jack has abandoned most of his school uniform and wears just a loincloth. He is obsessed by hunting and eventually discovers that painting his face helps disguise him from the pigs; it also helps move him to savagery. He look at at his reflection in the water and is astonished. He is looking
no longer at himself but at an awesome stranger. He spilt the water and leapt to his feet, laughing excitedly. Beside the pool his sinewy body held up a mask that drew their eyes and appalled them. He began to dance and his laughter became a bloodthirsty snarling. He capered toward Bill, and the mask was a thing on its own, behind which Jack hid, liberated from shame and self-consciousness.
Even the other boys are frightened at what they see when Jack dons the mask, subconsciously understanding that Jack with his mask on will feel no pangs of conscience for anything he does while wearing it. Without law or shame, Jack has no accountability to anything but his own desires and wishes.
Throughout the course of the novel, Jack grows increasinly hostile toward Ralph, who is the primary threat to Jack's authority on the island. Jack is the leader of the group chanting in a circle when Simon is killed, and he tacitly approves of Roger's killing of Piggy. Simon, the soul or spirit of man, is gone; and Piggy, the epitome of man's intellect, is also gone. All that stands between Jack and his object--complete control of the island--is Ralph, the last physical barrier keeping Jack from doing anything he wants to do. So, as chief of the tribe, he orders Ralph to be killed. A fire gets out-of-control and Ralph (because he represents the physical nature of man) is able to evade the savage hunters.
Jack is a terror on the island, threatening and taunting Piggy because what Jack wants to do is not reasonable; Jack's actions on the island are selfish and self-absorbed, and he truly cares for nothing but his own wants and needs.This is the aspect of human nature, Golding appears to say, which dominates the others.
As the naval officer who comes to rescue the boys looks at the group, he is particularly struck with Jack:
A little boy who wore the remains of an extraordinary black cap on his red hair and who carried the remains of a pair of spectacles at his waist, started forward, then changed his mind and stood still.
It is clear that, when confronted by the rule of law and authority (which the officer represents), Jack backs down. Human nature is restrained by the presence of law and authority; without that presence, human nature quickly devolves into evil.