In the beginning of Lord of the Flies, Ralph and Jack are in competition for the position of chief leader. In chapter one, we first learn about Jack's characteristics:
[Jack was] “tall, thin, and bony; and his hair was red beneath the black cap. His face was crumpled and freckled, and ugly without silliness. Out of this face stared two light blue eyes, frustrated now, and turning, or ready to turn, to anger.”
This is the first insight we have into the nature of Jack. He seems frustrated and angry. Jack's leadership skills are evident when he realizes there are no adults on the island. He states that they will have to take of themselves. Next, Jack tells Piggy that he is talking too much. We see that Jack is rude and inconsiderate. Jack is the first to suggests that he be named team leader.
After the boys vote, Ralph is chosen leader. The boys seem to prefer Ralph over Jack:
The boys appear naturally drawn to Ralph’s stillness, attractiveness, and the fact that he possesses the conch that summoned them.
Ralph appoints Jack as the leader of the choir. Jack's group becomes the hunters.
Ralph is diplomatic while Jack is rude and angry. When Piggy desires to explore the island with Jack, Ralph and Simon, Ralph tries to diplomatically discourage it while Jack blurts out “We don’t want you.”
Clearly, Ralph and Jack have different personalities. Golding seems to relate that "only Ralph seems remotely suited for his position. He seems to have at least some leadership ability." Golding also presents Ralph as attractive and charismatic. Golding feels that Jack's "physical unattractiveness and harsh ways prevent him from being elected."
Jack's actions also reveal his savagery. He has a temper. He slams his knife into a tree trunk when he cannot find the courage to kill the first pig he encounters. He chillingly vows that he will kill it the next time. Later on, Jack becomes comfortable with the bloody torturing of a sow. He laughs when Roger twists his spear in the anus of the sow. The sow screams in agony and Jack is amused:
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.
When Jack paints his face, he begins a silly dance that evolves into a "“bloodthirsty snarling.” Jack is becoming more and more aggressive. He is becoming dangerous. He leads the boys to kill Simon. He supports Roger who sends Piggy to his death by pushing a boulder down the mountain towards Piggy.
Although Ralph continues to represent responsibility, he is losing control. Jack has divided the boys. Ultimately, Ralph is running for his life. Jack and his hunters chase Ralph, desiring to kill him.
Fortunately, the Captain rescues the boys.