Jack has wanted his own tribe from the very beginning, when at the first assembly he argues that he should be chief because he is "chapter chorister and head boy" and "can sing C sharp" (22). From that moment, Jack has decided that the position should be his and views Ralph as merely an obstacle in his way to achieving it.
Jack uses the conflicts on the island and within the tribe to strengthen his position for becoming the next chief. He uses the boys' fear of the beast to intimidate them and then offers his protection as a hunter to reassure them. In "Gift for the Darkness," Jack has his final falling out with Ralph; in their highly public argument (all of the boys are watching), Jack accuses Ralph of saying that the hunters are no good and that they were cowardly; he twists Ralph's words to turn public opinion against his opponent. What Jack really wants is a re-vote for chief. When the other boys do not automatically jump to Jack's side and instantly vote him in, Jack leaves the tribe in a huff, declaring: "I'm not going to play any longer. Not with you" (127).
He then lures the other boys to his side with promises of hunting, no responsibility, and protection from the beast. His new base of power, Castle Rock, is a symbol of Jack's new authority, thus replacing Ralph and the conch.