Jack changes by going from an insecure and reasonably polite boy to a ruthless and violent one.
In the beginning of the story, Jack is a leader. He is choirmaster, so he naturally feels as if he should be in charge of the choir. The choir is used to listening to him, and he becomes the leader of this de-facto hunting group.
Jack is immediately offensive, calling Piggy “fatty” and arguing that he should be called “Merridew” because “Jack” is a kid’s name. Jack is never accepting of Ralph’s leadership, but becomes more and more of a bully.
“I ought to be chief,” said Jack with simple arrogance, “because I’m chapter chorister and head boy. I can sing C sharp.” (ch 1, p. 28)
When the boys try to choose who will be in charge, Jack is “the most obvious leader” (ch 1, p. 28).
The choir votes for Jack, and Ralph determines that they belong to him. Jack is unsure of himself at this point, but he begins to savor being in charge and eventually makes a coup against Ralph. He loves the idea of making rules.
Jack’s first love is hunting, but it does not start out that way. When Jack first catches the pig, he is unable to stab it.
He noticed that he still held the knife aloft and brought his arm down replacing the blade in the sheath. Then they all three laughed ashamedly and began to climb back to the track. (ch 1, p. 41)
Jack is not devoid of intelligence though, and he grows to be more self-sufficient. He has the idea of exploring the island looking for food. He also notices that Piggy’s glasses can be used to start the fire.
Jack's savagery symbolizes the potential in all of us. Jack does not start out as a violent person, but he is drawn to violence. He does not want order or civilization, he wants rules.