They knew very well why he hadn’t

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 is just a boy, who has never killed anything before. The actual act of cutting into a living 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 cannot kill a pig, never mind Pig-gy. However, as the novel continues, Jack’s trajectory moves steadily toward murder and brutality.