At the end of Chapter 1, the boys who have gone exploring--Ralph, Jack, and Simon--find a piglet stuck in the creepers in the jungle. Jack raises his knife but hesitates long enough for the pig to escape. Golding writes that they knew the reason Jack didn't let the knife fall: "because of the enormity of ... cutting into living flesh; because of the unbearable blood."
The fact is, Jack showed mercy on the screaming piglet. What caused him--and the other boys as well--to spare the piglet was no doubt a measure of empathy. The boys have just surveyed the island from some of the highest vantage points and have come to the realization that "there's no village smoke, and no boats." They are on an uninhabited island. They have been transported away from their parents--a similar state the piglet now finds itself in after the "hard strike of hoofs on a path" probably made by its fleeing mother. Even if the boys couldn't have expressed the connection they felt to the piglet, it certainly would have resonated on a subconscious level. The boys are stuck in a predicament with no assistance from adult caregivers, just like the baby pig. The boys are young--not so immature as the piglet, but young enough to remember a time not so long ago when they cried for their mothers when they were scared.
After allowing the the pig to escape, Jack seems to feel that showing mercy was a sign of weakness. He vows that will not be the case again. Despising rather than valuing his ability to show mercy to a helpless creature is part of what allows Jack's descent into violence and murder.