Why does Jack become so obsessed with killing a pig ?

Jack is obsessed with killing period. The pig is his first animal to conquer. Early in the book he was humiliated when he had a chance to kill the pig but didn't. He then becomes obsessed with proving his toughness and simply imposing his will on another living creature.

He was humiliated by something. I forget...hahahahahahahahahahaha.

He becomes obssesed with killing the pig because at first thee pig got away, and that embarrassed him. Jack relies on dominance, when he is embarassed by letting the pig get away he feels powerless and vulnerable, so by killing the pig he is proving a point and being the dominant one.