Jack calls an assembly in chapter 8 after Ralph, Jack, and Roger see the "beast" at the top of the mountain. He begins by telling the group of boys that they have seen the beast and that they couldn't kill it. Jack then immediatley begins to criticize Ralph by saying that Ralph thinks the "hunters are not good" and that Ralph thinks they are cowards. Jack tries to undermine Ralph's authority and position as cheif by telling the boys that Ralph is actually the coward because he was too afraid to go with Jack and Roger to the top of the mountain, which is a lie. In fact at the end of chapter 7, when the boys were on the top of the mountain, Ralph was hesitant at first, but he was the one who stood up and "took two leaden steps forward" toward the beast.
Jack lies to the boys because he realizes that he does not have as much power as Ralph, and he wants to make Ralph look bad in front of the other boys. Jack wants the boys to respect him more than Ralph.