What effect does hunting have on Jack ? Do you think the island changing Jack, or is Jack's true nature simply emerging?

The hunt seems to release all of Jack's inhibitions.... he becomes savage and dangerous. I believe the answer to your second question is both..... Jack obviously loves the hunt and loves the danger. If they hadn't been stranded on the island, Jack likely would never have experienced the hunt to this degree, although, he might have pursued the hunt as a sport later in life. As far as his nature is concerned, Jack obviously possesses different survival skills than Ralph. He's a rough and tumble boy..... fearless. None-the-less, his barbaric behavior is likely due to his circumstances rather than some inherent need for violence. Note.... my opinion.