An important quote about Jack being unable to kill the piglet is the following:

They knew very well why he hadn’t: because of the enormity of the knife descending and cutting into living flesh; because of the unbearable blood.

At this point, none of the boys, not even Jack, is willing to cross the line into killing, even though they are all hungry and need the food. They have all been socialized to avoid shedding blood, as the words "unbearable blood" indicate. Jack's socialization into British norms is as strong as that of the others, and it will take some time for him to shed his background.

A second important quote describes Jack's response to his moment of hesitation:

He snatched his knife out of the sheath and slammed it into a tree trunk. Next time there would be no mercy. He looked round fiercely, daring them to contradict.

Jack is angry at himself over not acting more ruthlessly. His thought that he won't make the same mistake again foreshadows the direction in which is he going to head. Jack, however, cannot manage releasing his id until he sheds his outward appearance as a civilized English boy. He will do this when he discovers the liberation he feels when he masks his face by painting it, which helps to free his inhibitions.

Golding is showing here that Jack is not innately a bad seed or somehow different from the other boys. As Simon will later discover, the beast the boys have to fear is the one within them, the will to evil that can be unleashed when civilized norms disappear.

Towards the end of chapter one, Ralph, Jack, and Simon are returning from their first expedition when they come across a piglet stuck in the creepers. Jack proceeds to brandish his knife in mid-air but hesitates to stab the defenseless animal, which gives the piglet enough time to escape the creepers and run away. Embarrassed by his inaction and lack of courage, Jack's face turns white underneath his freckles and he makes an excuse by telling the others he was deciding the perfect place to stab. Despite his excuse, Golding reveals the real reason Jack did not kill the pig by writing,

They knew very well why he hadn’t: because of the enormity of the knife descending and cutting into living flesh; because of the unbearable blood.

Golding's quote illustrates the significant influence of civilization on Jack and the boys. The boys have been conditioned by society to avoid violence and blood. Stabbing and killing a defenseless pig is a violent act, which is unlike anything they have ever experienced. Growing up in Britain, the boys were raised in a structured, organized environment with rules and regulations. On the tropical island, they are left to their own devices and unaccustomed to life in the wild. Their natural inclination to avoid "unbearable blood" and committing a brutally violent act emphasizes their innocence and civility.

As the novel progresses, Jack's hesitancy eventually disappears and the boys' civility rapidly diminishes. After Jack successfully makes his first kill, he develops a bloodlust and succumbs to his instincts. By the end of the story, anarchy reigns supreme as murder and violence are normalized under Jack's leadership.

Jack's inability to kill the blood of the piglet caught in the creepers indicates that, at this time, the vestiges of civilization are still stronger than the innate savagery in him.

As Ralph, Simon, and Jack explore the island in Chapter One, they happen upon an area that is covered with dark bushes with aromatic little buds. As they push farther into the forest, the boys hear the striking of hard hoofs and the squeals of a piglet who has become caught in the "curtain of creepers." The piglet writhes and hurls itself in terror, as it feels threatened by its entrapment. Jack rushes forward with the other boys and draws his knife "with a flourish," then pauses.

The pause was only long enough for them to understand what an enormity the downward stroke would be. (Chapter One, p. 23)

This pause later embarrasses Jack, who turns white from fear as he realizes the enormity of killing a living creature and spilling its blood.

In Lord of the Flies by William Golding, Jack catches a piglet tangled in vines in a scene from the first chapter.  When the piglet wiggles free and escapes, Jack does not want to be thought of as being weak or indecisive; the opinions of the other boys matter to him immensely.  He must save face by explaining himself:

"I was choosing a place," said Jack. "I was just waiting for a moment to decide where to stab him."

You should stick a pig," said Ralph fiercely. "They always talk about sticking a pig."

"You cut a pig's throat to let the blood out," said Jack, "otherwise you can't eat the meat."

"Why didn't you - ?"

They knew very well why he hadn't: because of the enormity of the knife descending and cutting into living flesh; because of the unbearable blood. (Ch. 1)

The bottom line is that all three boys feel squeamish about the thought of killing, and particularly of the blood.  Golding uses the word “enormity” in this quote to capture the importance of the boy’s taboo against killing; they know that they will eventually have to hunt to survive, but the aftermath of the blood is an unattractive prospect.  The blood makes the killing more real and less like a game.