Why does ralph crack the skull of the lord of the flies? what does this symbolize?

The Lord of the Flies - - the pig's head on the stick - - symbolizes evil. This "Beast" prompts the boys to give into their dark, savage sides.

When Ralph cracks the skull of the lord of the flies, he returns the creature to being merely a pig's head on a stick. He destroys its symbolic power. No longer a Beast, the Lord of the Flies is just a pig's skull. Ralph's act symbolizes law and order winning the battle over evil.