How did Jack change in Lord of the Flies?

At first he is pretty calm, and he just wants to fight to be leader. Though, soon he realizes that everyone has elected Ralph, he accepts it. He slowly pushes farther and farther until he finally gets everyone on his side, except Piggy, Samneric, and Ralph, making him the majority's leader. As soon as he gets power, he loses complete focus, and only feels the desire to kill and hunt, leading to Simon's death, and not the desire to get off the island.