In the beginning of the book, Jack is just a young man who has attended a school for boys and who is stranded on an island and as insecure as the other boys. Jack demonstrates leadership qualities in the beginning when he explains to the group that they have to come up with a plan to be rescued.
However, as time passes and Jack learns that he is the more physically dominating one, he begins to enjoy a sense of power. He becomes aggressive, dangerous, and is willing to kill.
Jack, at the first official gathering of the boys, demonstrates a desire for control over the group by appointing himself chief. However, the others call for an election. Despite the election, he likes Ralph.
"Jack and Ralph smiled at each other with shy liking."
Jack likes the adventure of exploring the island, but as he becomes less socialized, he starts to change into a raw savage state of being. However, following the pig hunt with Ralph, he is still Ralph's friend.
Jack becomes more savage, controlling the others with his physical and mental dominance. Ralph becomes distant from Jack because he has not followed into Jack's level of savage behavior.
"Numberless and inexpressible frustrations combined to make his rage elemental and awe-inspiring."
Ralph and Jack separate into two groups with Jack refusing to have anything to do with Ralph's group. Jack as a leader does cruel things.
"He got angry and made us tie Wilfrid up."
"He's going to beat Wilfrid."
The twins inform Ralph that Jack is going to kill him like in a pig hunt. Jack has transformed to evil.
"The savage peered into the obscurity beneath the thicket."