This is an interesting question, because what Jack is in the beginning of William Golding's Lord of the Flies both juxtaposes and foreshadows what he is at the end of the novel.
When we first meet him, Jack is an imperious, red-headed boy who begins making demands from the first moment he speaks. He is the head of a choir and seems to rule them quite severely. They are not allowed to even sit on the beach until he tells them they can (after they begged him to allow it), and we know they do not like Jack's leadership because they do not want to vote for him to be leader on the island (though they do, under his pressure.)
Jack is immediately disdainful and cruel to Piggy for no other reason than Piggy's appearance, and he dismisses anything Piggy says, even though Piggy has the conch. It is interesting that Jack is the only boy on the island who was carrying a knife at the time of the plane crash; though he brags about using it, the first time he has the opportunity he is unable to do it. That will soon change.
As Jack's obsession with hunting grows, so does his cruelty. Eventually he discovers that he can catch pigs if he paints his face; when he does, his transformation to "savage" is nearly complete. As Jack looks at his painted-face reflection, something changes.
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. He capered toward Bill, and the mask was a thing on its own, behind which Jack hid, liberated from shame and self-consciousness. The face of red and white and black swung through the air and jigged toward Bill. Bill started up laughing; then suddenly he fell silent and blundered away through the bushes.
Whatever Bill, one of Jack's choirboys, saw in Jack at that moment appalled him and caused him to run away. Jack now has the complete freedom, because he feels no shame or guilt, to do whatever he wishes--and Bill clearly knows what that might mean.
Jack becomes the chief of a tribe of savages, and he moves form killing pigs to killing humans. He allows Piggy's murder and he tries to stab Ralph with his spear. When Ralph escapes, Jack orders his savages to hunt him down and kill him--which they would have done if the naval officer had not appeared on the beach.
Jack's transformation is not from a wonderful, polite little boy into the chief of a tribe of savages; instead, his transformation is more of a dramatic enhancement and magnification of the same cruelties he displayed at the beginning of the novel. That is foreshadowing.
The juxtaposition occurs when we see Jack through the eyes of the naval officer at the end of the novel. He looks at Jack and sees this:
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.
This is not the same brash, dismissive, belligerent leader of the choir we met in the first pages of the novel. Though he is the same, he is also different.
At the beginning Jack is represented as an innocent but controlling school boy who thought that he should be leader purely because he could sing well. Uniformity and autority was all that mattereed to him. However as the novel progressed he developed an obsession to kill a living creature in this case a pig. Later on his character became increasingly conflicted as his conscience battaled with his savage obsession. Until in the end he was immune to his conscience, wearing a mask to become someone else and detach himself completely from civilisation and his previoud life.