In the beginning of William Golding's Lord of the Flies, Jack wants to be leader. While at the meeting after killing the pig, he wants to forget about rules and leaders and just hunt. Why does this change in Jack occur?
It is true that Jack Merridew thinks he should be elected leader at the first meeting the boys hold in Lord of the Flies by William Golding; it is also true that the only people who vote for him, reluctantly, are his choir. If he had been elected leader, he may have been more interested in maintaining order and leadership, at least for awhile; however, from the moment he becomes chief of the hunters, Jack has virtually no interest in anything but hunting.
In chapter three, Ralph and Simon have been working hard to build the shelters, as the group decided. Jack, on the other hand, has been hunting all day and has no desire to help once he is finished hunting. He says he hunts because the tribe needs meat, but it is clear that his obsession would compel him to hunt whether or not there was meat involved. He and his hunters are charged with keeping the signal fires lit, but they fail miserably at their task. An argument can be made that Jack never did pay any serious attention to leadership and rules unless it suited his own interests.
Your question asks why the change occurs; in chapter four, Jack realizes he has not been successful in catching a pig because the animals see him. He decides to paint his face, and that is when Jack's actions and motivations change. He looks at himself in some water.
He looked in astonishment, no longer at himself but at an awesome stranger. He spilt the water and leapt to his feet, laughing excitedly. Beside the pool his sinewy body held up a mask that drew their eyes and appalled them. He began to dance and his laughter became a bloodthirsty snarling. He capered toward Bill, and the mask was a thing on its own, behind which Jack hid, liberated from shame and self-consciousness.
Jack changes because he no longer feels any natural restraints, such as shame and conscience. From this point on, Jack is obsessed with hunting (killing) like never before; later in the chapter he kills his first pig--and the killing begins.