Why does Achilles spare twelve young men in the river?

Achilles routs the Trojans and splits their ranks, pursuing half of them into the river known to the gods as Xanthus and to the mortals as Scamander. On the riverbank, Achilles mercilessly slaughters Lycaon, a son of Priam. The Trojan Asteropaeus, given fresh strength by the god of the river, makes a valiant stand, but Achilles kills him as well. The vengeful Achilles has no intention of sparing any Trojans now that they have killed Patroclus. He throws so many corpses into the river that its channels become clogged. The river god rises up and protests, and Achilles agrees to stop throwing people into the water but not to stop killing them. The river, sympathetic to the Trojans, calls for help from Apollo, but when Achilles hears the river’s plea, he attacks the river. The river gets the upper hand and drags Achilles all the way downstream to a floodplain. He very nearly kills Achilles, but the gods intervene.