Why does the piglet trapped in the creeper vines escape in lord of the flies?

Jack does not kill the piglet in chapter one because he is just a boy. He was the head boy in a choir, he probably came from a priviledged background, and had never had to kill anything before in his life. It is one thing to say "I will a pig" but it is another thing entirely to stab a squealing terrified young animal to death, to feel the knife slicing through living flesh. It is hardly surprising that in those circumstances Jack baulked and failed to stab the piglet. Because of this understandable hesitation on Jack's part the piglet's frantic struggling frees it from the creepers and it then escape.