Achilles' face turned sour. This child (and he was an arrogant child!) held no remorse for the lives he'd taken. He unlike Achilles had no argument for killing other than he enjoyed it. Achilles had compassion on Priam when he had come into the enemy lines to beg for his son's body. Achilles had been through many wars, but in the end he would feel guilty. Here he continuously struggled with needing to be a soldier and putting it into a healthier outlet.
It was sad to see what had become of his own son. "Did you enjoy that? Ruthlessly taking life just for entertainment?" He could see now Neoptolemus was a the monster inside him.