It's evident, from early on in the novel, that Jack and Ralph have very different approaches to surviving on the island. Ralph, who is driven by his need for order and his desire to get off the island, insists that the signal fire is the most important thing on the island--that they have no hope of being rescued without it.
On the other hand, Jack, who is in a constant struggle for power with Ralph, argues that finding food is the most important thing that the boys can do while they're on the island. He seems to understand that the boys' rescue is not immediately impending, and insists that tey must have meat in order to survive.
Upon closer examination, though, readers come to understand that Jack's desire to hunt is not driven by the need to provide food for the boys on the island. Instead, Jack is possessed by an instinctual desire to kill. In chapter three, Jack is briefly unable to articulate his ideas because he is so consumed by his desire to get a pig:
He tried to convey the compulsion to track down and kill that was swallowing him up.
It is this instinct (Freud would describe it as the Id) that consumes Jack and makes him forget about everything else. He lets the signal fire go out, disregards the rules and order Ralph has worked so hard to establish, and is ultimately so overtaken by the desire to hunt that he kills humans. In short, Jack is the boy whose descent into savagery is the fastest. It's not necessarily that he doesn't want to be rescued, as he never explicitly states this; it's that his psyche is overpowered by the savage instinct that Golding thinks exists in all of us but is controlled by rules imposed by society.