Jack plans a raid on Ralph's camp to get the one thing that they don't possess: Piggy's glasses. The reason that they don't "ask" as Ralph later yells at Jack, is because Jack and his tribe are too far gone into the lifestyle of savagery. They no longer possess any civilized qualities. Jack wouldn't just ask for them or to borrow them; he needs to exert his "power" over the others in his tribe and go raid the other camp and steal what he needs. That is who they have become, and they will no longer be playing "nicely" together. This is he chapter that follows Simon's murder/death. Next will be Piggy's death, which is why Jack and his tribe are in the position they are in.