From the outset, we become aware of Jack's desire to be noticed and to lead. He deems himself the superior of other boys and does not see himself as a child, as indicated by his response when Ralph suggests that they should all have names and uses his first name:

“Kids’ names,” said Merridew. “Why should I be Jack? I’m Merridew.”
Ralph turned to him quickly. This was the voice of one who knew his own mind.

Ralph instinctively realizes that Jack is going to be difficult to deal with. When the boys later decide on Ralph as chief, Jack is clearly upset, but Ralph diplomatically arranges a compromise and asks him to lead the hunters. At this point, there is a "shy liking" between the two boys.

Jack is initially supportive of having rules and creating an environment in which everyone is respectful and dutiful. In chapter two, when Ralph suggests that they should have "hands up" like at school, he is excited about having rules and shouts:

“We’ll have rules!” he cried excitedly. “Lots of rules! Then when anyone breaks 'em—”

Jack's response hints at something sinister and malicious within him, though. His incomplete sentence suggests punishing those who break the rules, something he would probably relish doing. At this point, he still asks for the conch or reaches out for it before speaking. He and Ralph work together collecting firewood once it the group decides to build a signal fire. When Ralph later again stresses the importance of rules, he is fully supportive of him and says:

“I agree with Ralph. We’ve got to have rules and obey them. After all, we’re not savages. We’re English, and the English are best at everything. So we’ve got to do the right things.”

He later suggests that his group will be responsible for maintaining a signal fire and that they will have a schedule.

“Ralph, I’ll split up the choir–my hunters, that is–into groups, and we’ll be responsible for keeping the fire going–”

However, once Jack becomes immersed in hunting, he forgets about his earlier promises and he and the hunters neglect their duties. Hunting pigs has become an obsession and they spend most of their time looking for prey. In chapter three, he and Ralph argue about this and he tries to explain his position:

“I went on,” said Jack. “I let them go. I had to go on. I—” He tried to convey the compulsion to track down and kill that was swallowing him up. “I went on. I thought, by myself—”

He becomes angry at Ralph and insists that he will kill a pig. The antagonism between the two becomes palpable at this point. The confrontation indicates the beginning of Jack's rebellion against the rules and a breakdown in their initial cordial relationship.

In addition, Jack forgets about rescue and has to be reminded. When Ralph tells him about it he responds by saying:

“Rescue? Yes, of course! All the same, I’d like to catch a pig first—”

When Ralph reminds him of the fire, he seems to alienate himself from it and talks about "you and your fire!" The two boys, from this point, seem to be always at loggerheads with each other.

In chapter four, Jack's obsession with maiming and killing leads to the fire going out at a most crucial time—when a ship is seen passing the island. Ralph is overwhelmed by the idea that they have missed an opportunity for rescue and confronts Jack about their disregard for duty. Jack lashes out and targets Piggy. He smacks him on the head, sending his glasses flying. One lense is broken and Jack apologizes later, somewhat soothing the damage done.

In chapter five, Jack's resentment for the rules he so avidly supported becomes violently apparent. He grabs the conch from Piggy and is admonished by Ralph. He responds by saying: "Who cares?" When Ralph tells him that rules are the only thing they have, he shouts:

“Bollocks to the rules! We’re strong—we hunt! If there’s a beast, we’ll hunt it down! We’ll close in and beat and beat and beat—!” 

Jack jumps down from the platform, making a loud whoop, and chaos ensues. All the other boys run off the platform and go in different directions. It is clear, at this juncture, that Jack has lost most rationality and has allowed his inner savage to start dominating his reason.

In chapter six, he expresses his disdain for rules and for the discipline that the conch symbolizes:

“Conch! Conch!” shouted Jack. “We don’t need the conch any more. We know who ought to say things..."

At this point, however, Jack is still somewhat cooperative and joins Ralph and the others to look for the beast. He sees this as an opportunity to hunt since he believes that if there should be a beast, he would be able to track and kill it. At the end of chapter seven, though, both he and Ralph are so alarmed at what they believe is the beast that they forget everything and focus only on getting away from the terror they have both seen.

Before them, something like a great ape was sitting asleep with its head between its knees. Then the wind roared in the forest, there was confusion in the darkness and the creature lifted its head, holding toward them the ruin of a face.

The sight of what they believe is the beast foreshadows the greater danger which Ralph has to face and predicts the exposure of the boys' true savagery when death and chaos become a reality.

In the beginning of the novel, Jack is very much tied to civilization.  He originally appears still dressed in his formal choir robes with the rest of the choir dutifully trailing behind him.  When debating about leadership and rules, Jack interjects:

"We'll have rules [...] lots of rules!  Then when anyone breaks 'em--" (33).

Jack demonstrates both his appreciation for rules and authority in the way that he controls the choir and uses his position as 'head chorister' to dominate the younger boys.  As the novel progresses, however, Jack loses his focus on the old, civilized world as he becomes enamored with hunting.  When he dons the paint for the hunt, he transforms into a savage hunter.  Jack's shift from choir boy to savage becomes even more apparent when he dismisses the authority of the conch:

"Conch! Conch!" shouted Jack.  "We don't need the conch anymore. We know who ought to say things" (101-102). 

Jack denies the very rules and authority that he used to whole-heartedly support.  He has shed his choir robes and replaced them with the paint and mask of a savage.  Rules and the authority of his former life have completely lost all meaning to him.