We meet Jack Merridew in the first chapter of William Golding's Lord of the Flies, as he leads his choir toward the sound of the conch. The characteristics he displays then are the same characteristics he demonstrates at the end of the novel; his cruel nature is simply magnified and intensified as the story progresses.
Jack is effective in keeping his choir under control, but he does so through intimidation. The fully robed boys follow Jack to the meeting place on the beach. "When his party was about ten yards from the platform he shouted an order and they halted, gasping, sweating, swaying in the ﬁerce light." Despite the boys' obvious discomfort, Jack orders them to "halt" but does not let them sit or take off their sweltering clothes until they beg him.
Golding describes Jack this way:
he was tall, thin, and bony; and his hair was red beneath the black cap. His face was crumpled and freckled, and ugly without silliness. Out of this face stared two light blue eyes, frustrated now, and turning, or ready to turn, to anger.
Immediately Jack demands to know who is in charge, and from the beginning, Jack taunts Piggy and calls him names, simply because of Piggy's physical appearance. He is an angry, frustrated boy. The group decides to elect a leader.
“I ought to be chief,” said Jack with simple arrogance, “because I’m chapter chorister and head boy. I can sing C sharp.”
When Ralph asks who wants to vote for Jack, no one responds immediately--not even the choirboys until "with dreary obedience the choir raised their hands." When Ralph wins this election, Jack is embarrassed, but Ralph appeases him by telling Jack he can have the choir as hunters.
Jack likes to explore, but he particularly likes to hunt. He is obsessed with hunting--the only thing which is important to him--and does nothing to make himself useful around the camp. While Ralph wants shelter, Jack wants meat.
The moment when Jack begins to transform in a significant way is when he begins painting his face to hunt. Something happens which turns Jack from a surly boy with a knife into a "savage." After he paints his face, he looks at his reflection in some water.
He looked in astonishment, 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.... [T]he mask was a thing on its own, behind which Jack hid, liberated from shame and self-consciousness.
Jack, like the others, has had no adult authority on the island; however, until now he has demonstrated the self-restraint of his conscience to keep from doing anything truly awful (though Piggy has known almost from the beginning that Jack would like to get rid of both Piggy and Ralph). Now, behind the mask, Jack has nothing to restrain him, and this is the beginning of his descent into savagery.
By the end of the novel, Jack is a leader again; he is the chief of a tribe of savages. He maintains control of the boys by threatening and intimidating them. Once Piggy and Simon are dead, the only impediment to Jack's complete control of the island based on his own self-centered, evil human nature is Ralph. When Jack orders his tribe to kill Ralph, no one doubts that he means it, including Ralph. Jack's tribe follows his orders, just as his choir did in the beginning.