His savagery developed because he was trying to survive
His character develops from being a reasonably good boy that was once a choirmaster into one who begins to have a distorted sense of reality on the island. His character shows what happens if the rules are taken away. Honestly, Jack is an example of the little savage in all of us.
His behavior was cemented in wanting to survive the island. He lost focus on the idea of being rescued because he was so focused on the idea of survival. Becoming stranded on an island as a young boy with no idea if you are going to found has to cause psychological stress. In a broader sense, we would consider Ralph to be representing the "good' while Jack is representing the "evil", but everyone in Lord of the Flies was "bad", and so is everyone on earth. People are instinctively savage, and that was William Golding's message. Appalled by wartime atrocities as well as war itself, he made Jack symbolize the barbaric nature everyone has when it comes to drastic measures. Even Ralph, the protagonist, and Piggy, who seemed wise and good-natured, surrendered to barbarism by helping to kill Simon. Golding symbolises the fact that subversive evil will survive, whereas any inch of goodness will be destroyed. Though Jack's behavior was extreme, no one in the novel was any better than he was (with the exception of Simon, who was inherently moral).
Survival on the island only played a small part in Jack's behavior. Though he was determined to survive the island, his obsession with authority and violence is what caused him to spiral out of control, not the need to survive. He was fixated on becoming chief from the very beginning, and Ralph being voted as chief instead is what caused his horrible behavior. If he was initially voted as chief, he more than likely would not have acted so aggressively.
[P1] His extreme behavior was due to a drastic situation.
Rejecting the premises
[Rejecting P1] He chose this behavior. All of the boys were exposed to the same environment as him and did not act this way.