In the beginning of the novel, Jack is confident and charismatic. He is most likely someone who is used to getting his own way. As he describes himself, "I'm chapter chorister and head boy. I can sing C sharp." Although Jack shows disdain for others--for example, he calls Piggy "Fatty" before knowing who he is--he is likable, and Ralph immediately wants to befriend him. Jack seems to need the approval of others; he is ashamed when he fails to kill the first piglet they run across. At first Jack tries to gain this approval by cooperation and by leading the hunters. He states, "We've got to have rules and obey them."
Whenever he isn't properly acknowledged, however, he tends to lash out. When caught in his huge error of letting the fire go out just when a boat was passing the island, Jack takes the reprimand from Ralph but lashes out at Piggy, charging into his stomach and breaking his glasses. From that point, Jack turns fully to hunting as a way to increase his status. Jack plays on the fears of the boys to disrupt a meeting and cause Ralph's leadership to falter. When Ralph and Jack and some other boys go out in search of the beast, Ralph becomes frustrated with Jack's self-centeredness: "Ralph sighed, sensing the rising antagonism, understanding that this was how Jack felt as soon as he ceased to lead."
Finally Jack attempts open rebellion, calling for a vote against Ralph. When it doesn't go Jack's way, Jack pouts and says, "I'm not going to play any longer. Not with you." This statement shows Jack's immaturity. Even as Ralph has become more mature, putting the needs of others before his own, Jack is becoming more caught up in his own feelings and desires. When Jack starts his own tribe, his descent into savagery hastens. He soon uses violence and theft to get what he wants, and he presides over the raucous feast that results in Simon's murder. He uses his newly gained "irresponsible authority" to intimidate, torture, and physically harm the other boys. Ultimately, he makes a plan to murder Ralph as if he were a pig.
Interestingly, at the end of the story when Jack appears before the naval officer who asks, "Who's boss here?" Jack "started forward, then changed his mind and stood still." Perhaps only at that moment, standing there as "a little boy" before a representative of adult authority, does he begin to question the depths to which his arrogance and misplaced self-confidence have led him.
Much of Jack's change in the novel is one of growth into his own personality. From the beginning he has a sort of rebellious streak, one that wants to go against what Ralph and Piggy set up but he is nervous about it. Just as he is afraid to really stab the pig the first time he is confronted with it, he is afraid to take action, particularly if it will hurt something or someone, to get what he wants.
As he grows throughout the novel, he loses this fear of hurt. He focuses far more on the way he knows to get power, to get what he wants, and develops quite a knack for it. His confidence grows incredibly quickly and he is more and more willing to hurt or to allow hurt in order to get what he wants.