How does Jack react to Piggy’s death and why?

After Piggy falls from the cliff, the group falls into a sudden and deep silence. Just as suddenly, however, Jack leaps out of the group, screaming deliriously. He shouts at Ralph that "that's what you'll get" for challenging his authority, and he expresses happiness that the conch is gone. Declaring himself chief, Jack deliberately hurls his spear at Ralph. The spear tears the skin and flesh over Ralph's ribs, then shears off and falls into the water. He does this to cement his position as chief.