Viciously, with full intention, he hurled his spear at Ralph. The pint tore the skin and flesh over Ralph's ribs, then sheared off and fell in the water. Ralph stumbled, feeling not pain but panic, and the tribe, screaming now like the chief, began to advance.
Jack shows his ultimate lust for power here when he attempts to kill Ralph with his spear. He tries to show his dominance by taking out the last bit of power on the island. Since he failed he now told his group to charge Ralph in an last attempt to kill him.