How does Jack react when the boys find the piglet?
Jack has, throughout, been slamming his knife into trees in a threat of violence. Yet the moment when the piglet actually appears demands from Jack actual violence rather than just threatened violence.
Jack lifts his knife and hesitates:
The three boys rushed forward and Jack drew his knife again with a flourish. He raised his arm in the air. There came a pause... only long enough for them to understand what an enormity the downward stroke would be.
Jack is worried about the "enormity" of the downward stroke. He is still a little boy, and at this early stage of the novel, has never killed anything.
Then the piglet tore loose from the creepers and scurried into the undergrowth. They were left looking at each other and the place of terror. Jack’s face was white under the freckles. He noticed that he still held the knife aloft and brought his arm down replacing the blade in the sheath. Then they all three laughed ashamedly and began to climb back to the track.
“I was choosing a place,” said Jack. “I was just waiting for a moment to decide where to stab him.”
Jack's reaction is to let the piglet go. The piglet runs away. And the boys look at Jack, wondering why he didn't do it. His response?
“I was choosing a place. Next time—!"
He snatched his knife out of the sheath and slammed it into a tree trunk. Next time there would be no mercy.