At the beginning of the novel, Jack's power was similar to that of the other boys since they were all new to the island and each other. The only dividing factor was that Jack had some additional power due to his position as leader of the choir boys.
Jack was a born leader and when he was not elected leader of the boys on the island he immediately sought ways to gain power.
Jack was elected leader of the hunters. This allowed him to make his first step, up the ladder of power on the island.
While exploring the island with Ralph and Simon, the boys encountered the first pig on the island. Jack, being the leader of the hunters, stepped in for his first kill but hesitated and couldn't bring himself to execute the trapped piglet. This showed that Jack was not corrupt at the beginning of the novel because he could not think of himself killing an animal.
On page 29 Ralph says, "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. William Golding made it very clear that Jack still valued his morals and believed that killing was wrong.
As the novel progressed, power began to corrupt Jack and now that he was the leader of the hunters, he killed pigs without hesitation.
Jack had progressively gained power on the island and began to hunt more regularly with his hunters.
Jack had gained more power at this point in the novel because he took Sam'n'eric away...