When Jack, who has been tasked to provide food for the "tribe" of boys on the island, first has an opportunity to kill a piglet, he cannot bring himself to slaughter the animal. When the boys talk about the hunt, Jack explains away his inability to kill the pig this way:
"I was choosing a place," said Jack. "I was just waiting for a moment to decide where to stab him."
But the other boys know better. They know the real reason that Jack could not kill the pig is
because of the enormity of the knife descending and cutting into living flesh; because of the unbearable blood.
Jack is humiliated by his self-defeat. Ralph has recently won out over Jack as leader of the boys, and Jack wants desperately to prove himself. He becomes even more determined to succeed:
"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. He looked round fiercely, daring them to contradict.
His promise of killing a pig "next time" is fulfilled, as he goes on to kill several pigs--and, indirectly, a lonely boy named Piggy.