An important quote about Jack being unable to kill the piglet is the following:
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.
At this point, none of the boys, not even Jack, is willing to cross the line into killing, even though they are all hungry and need the food. They have all been socialized to avoid shedding blood, as the words "unbearable blood" indicate. Jack's socialization into British norms is as strong as that of the others, and it will take some time for him to shed his background.
A second important quote describes Jack's response to his moment of hesitation:
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.
Jack is angry at himself over not acting more ruthlessly. His thought that he won't make the same mistake again foreshadows the direction in which is he going to head. Jack, however, cannot manage releasing his id until he sheds his outward appearance as a civilized English boy. He will do this when he discovers the liberation he feels when he masks his face by painting it, which helps to free his inhibitions.
Golding is showing here that Jack is not innately a bad seed or somehow different from the other boys. As Simon will later discover, the beast the boys have to fear is the one within them, the will to evil that can be unleashed when civilized norms disappear.