In the final chapter of Lord of the Flies, by William Golding, every boy on the island--except for Ralph--is now part of Jack's tribe. The last two to join were Samneric; though they would rather have stayed with Ralph, they are forced to join at spear-point.
Everything on the island changes when Piggy is murdered and the conch is destroyed. As the twins are coerced into the tribe of savages, Ralph begins running for his life. Later the twins tell Ralph that he is being hunted, and we will hear Jack give specific orders for his tribe to kill Ralph. In addition to that, though, Ralph instinctively understands that Jack needs to kill him.
As he reasons with himself, hoping that this is all just a misunderstanding, Ralph finally admits the truth:
He argued unconvincingly that they would let him alone, perhaps even make an outlaw of him. But then the fatal unreasoning knowledge came to him again. The breaking of the conch and the deaths of Piggy and Simon lay over the island like a vapor. These painted savages would go further and further. Then there was that indeﬁnable connection between himself and Jack; who therefore would never let him alone; never.
Ralph has correctly identifies the two reasons why Jack must kill him. First, the unrestrained savagery on this island is only going to get worse, and anything that might impede it (such as Ralph's reasoning) must be destroyed. Second, Jack has held a grudge against Ralph from the first moments they met, when Ralph was elected chief and Jack was not. In his increasing savagery, Jack must ensure that Ralph does not live.
because Jack realises that he cannot have complete power over the island while Ralph is still alive as at anytime Ralphwill always be a threat to his authority