In Lord of the Flies, William Golding presents the relationship between Ralph and Jack as strained, uneasy, and complex before it breaks down into absolute hostility. In chapter 3, a long argument about what their priorities ought to be ends with the following passage:

They walked along, two continents of experience and feeling, unable to communicate.

“If I could only get a pig!”

“I’ll come back and go on with the shelter.”

They looked at each other, baffled, in love and hate. All the warm salt water of the bathing pool and the shouting and splashing and laughing were only just sufficient to bring them together again.

In the dialogue that precedes this, Ralph has been arguing for the necessity of building shelters, while Jack has been talking about killing a pig. This shows the way in which Ralph emphasizes practical, rational governance, and the creation of an ordered society, while Jack seeks visceral satisfaction. The metaphor of the two continents reinforces the idea of the division between them.

Golding's comment about love and hate, however, emphasizes that there is a sense in which the two boys are similar. They are both strong personalities and natural leaders. Ralph sees that he has more in common with Jack than he has with Piggy, despite the fact that Piggy is clearly a more decent and positive personality than Jack. He respects and even admires Jack's strength of will. Under the constraints of civilization, the two strong characters might have enjoyed a relatively friendly rivalry. On the island, however, the worst elements of their natures are free to cause havoc.

The titanic struggle for supremacy between Ralph and Jack in Lord of the Flies isn't simply a battle between two boys but between two radically different elements of the soul.

Ralph represents that part of the soul that is rational, ordered, and logical. Whereas Jack represents the dark, violent, disordered depths of the human soul, that part of the soul that lurks within each and every one of us but which most of us thankfully manage to keep in check.

Of all the boys on the island, only Simon seems to understand the soul's darkest depths. He realizes all too well that there's nothing remotely unusual about the kind of violence in which Jack and his gang routinely indulge. It's an expression of that part of the soul which tends to emerge under conditions of extreme stress and when the normal rules by which we live begin to break down.

Ralph and Piggy think they can build a civilization from scratch on the basis of reason, democracy, and good order. But though their intentions in this regard are nothing but admirable, they display naivety in failing to establish some kind of force that can keep in line those who don't want to live under a rules-based system, who want to do their own thing.

As a consequence, Jack is able to give free rein to the dark side of the soul, with its desire for chaos, disorder, and violence. What's more, in his epic struggle with Ralph, he allows other boys to do the exact same thing, to give them permission to indulge in violence, bullying, and savage rituals.

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.