Why does jack hesitate to kill the piglet in the lord of the flies?

Because he doesn't like him

Its sort of a Logic and Intelligence(Piggy) vs. Bloodlust and Savaregy(Jack) thing that Golding might've thought of.

Basically Jack wants to have power and be the leader. At a time when he feels that he has just proven his prowness as a hunter Ralph has reasserted his own leadships over the incident of letting the fire out. Jack has already punched Piggy, who he dislikes but also sees as an easy target and a way of getting at Ralph. Jack then refuses to give Piggy any meat as a further display of his own power. Jack killed the pig and Jack chooses who eats it, not Ralph.