how has jack changed?

Jack certainly changes the moment he realizes that there are no adults on the island. It would seem that something clicks in his little brain to unleash his inner socio-path. In chapter one Jack is still cognoscente of the old world. He allows a vote for chief and, when he loses, merely stabs trees with his knife. It's certainly better than stabbing Piggy or a Littlun! By chapter 3 Jack has become obsessed with killing a pig. Sure they need meat but Jack's obsession is more about the act of killing than the acquisition of protein. He paints his face and realizes the release it gives him from his old life. The old Jack merridew is quickly developing into the creepy socio-path that we meet by the end of the novel.