Jack is allegorical because he represents the innate savagery of human nature.
Allegory means something stands for something larger than itself. An allegorical story provides a lesson for society. An allegorical character is one we pay attention to because there is a lesson in him. The lesson Jack has for us is that we have to look out for the savage in ourselves.
From the beginning, Jack serves as an alter ego of Ralph. Ralph is the one who represents order and the civilization from which the boys come. Jack is the chaos and savagery into which they will descend.
When Ralph is first elected leader, Jack is his immediate competitor. He has a built-in constituency because he is the choir leader. Ralph wants to keep order, and Jack just wants to control others. At the first meeting, Jack’s reaction to Ralph’s attempts at creating order are ominous.
Jack was on his feet.
“We’ll have rules!” he cried excitedly. “Lots of rules! Then when anyone breaks ’em–” (Ch. 1)
Ralph wants rules so that everything is kept civilized. Jack wants them so that he can punish anyone who breaks them.
Jack is a natural leader. Ralph is only elected because of the symbolic power of the conch. Yet Ralph's desire to keep order comes from a desire to keep everyone safe. Jack's desire to be leader comes from a need to control.
Jack and Ralph are at odds from the start. Ralph is cognizant of the fact that if he slips, Jack will be there to pick up the pieces. He tries to keep Jack at bay by offering him and his choir a role as hunters, but it turns out to be exactly the wrong move. It reduces Jack to his more savage instincts, and slowly Ralph loses whatever semblance of control he had over him and his tribe.
Ralph is intent upon one thing—rescue and returning back to civilization. He wants to keep the signal fire going so they can get rescued. Jack is more interested in hunting, not just for food (they have fruit too, so they do not desperately need it), but because it returns them to their baser instincts, their savage ways. It keeps them at odds.
“Rescue? Yes, of course! All the same, I’d like to catch a pig first—” He snatched up his spear and dashed it into the ground. The opaque, mad look came into his eyes again. Ralph looked at him critically….
“So long as your hunters remember the fire—”
“You and your fire!” (Ch. 3)
To Jack, the siren song of the hunt is more important than getting rescued. Civilization is no longer as important. As Jack gets deeper and deeper into this unrestrained version of himself, bringing most of the other boys with him, he represents it physically with paint and the pig dance.
By the time Jack has separated from the rest of the tribe, he is unrecognizable.
He spun on his heel, center of a bewildered circle of boys.
“I got you meat!”
Numberless and inexpressible frustrations combined to make his rage elemental and awe-inspiring.
“I painted my face—I stole up. Now you eat—all of you—and I—” (Ch. 4)
It is this savagery that causes Piggy’s death (and Simon’s), because the boys have forgotten their ties to civilization and the need to stop and think before acting. When they mistake a boy for a beast, they are doing so out of desperation and anxiety driven by their disconnection from the world they have left. This is Jack’s world, not Ralph’s. This the savage side of them.
The lesson we can learn from Jack is that we all have the potential for chaos, and possibly evil. There is a fine line between the civilized self that most of us have and the savage one that we could be. Jack allowed himself to slip into the savage quickly and easily. How easy would it be for you?