In Lord of the Flies, Jack has always envied Ralph's position as the leader. In fact, Jack longed to be the leader. He lost the votes and Ralph was put in charge. Jack never respected Ralph as the leader. When Ralph asked Jack to keep the fire going, Jack ignored his request and went hunting.
Truly, Jack is trying to divide the boys. He is trying to get all the boys to turn against Ralph. As Ralph's civilized world disintegrates, Jack becomes in charge. His evil ways are taking over the others. Jack has become a savage. Jack separates his group from Ralph's group. Eventually, Jack convinces all the boys to follow him:
Then Jack sets about wooing away the other boys to his group.
No doubt, Jack was jealous of Ralph and all that Ralph stood for. Jack would not help Ralph build the shelters. Jack became a blood thirsty killer. He lost all respect for even humanity. Jack led the boys in killing Simon. He supported Roger in his inhumane torturing of the sow:
Jack begins to rub the blood on his hands onto Maurice, and then they notice Roger withdraw his spear. They become hysterical because he had pinned the sow by driving the spear through its anus.
Jack is cruel and even dangerous. He will stop at nothing to gain total leadership over all the boys. Little by little, he has gained the support of all the boys, even though he did it through terror. Ralph is the only one left who has not submitted to Jack's authority. That leaves Jack with only one solution. He must kill Ralph. Roger has sharpened a spear on both ends in which to hang Ralph's head. Ralph is running through the forest trying to escape being killed by Jack and Roger. Jack has become destructive. He has lost all regard for human life. He will do whatever it takes to be totally in charge. He is a killer. He enjoys killing. His evil instincts have surfaced:
Jack was the antithesis of Ralph, yet mirrored Ralph’s inability to see the other side of his nature. He was violent and anarchic, and sought rule by absolute power. His belief in his own authoritarian rules would have eventually lead to the destruction of them all.
Fortunately, the naval officer saved Ralph's life.