In "Lord of the Flies", why does Jack hesitate at his first opportunity to kill a pig?

This is the bit from the novel you need, from the end of the first chapter:

Jack drew his knife again with a flourish. He raised his arm in the air. There came a pause, a hiatus, the pig continued to scream and the creepers to jerk, and the blade continued to flash at the end of a bony arm. The pause was only long enough for them to understand what an enormity the downward stroke would be. 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....

“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.

“I was going to,” said Jack.

Jack pauses because he's just a boy, he's never killed anything before, and the actual act of cutting into a live creature with a knife was just too much for him. The blood which followed would be "unbearable". So Jack hesitates, and the pig escapes.

It's a clear sign from Golding that Jack doesn't start off as a monster. He can't kill a pig, never mind Pig-gy. But, as the novel continues, Jack's trajectory moves steadily toward murder and brutality.