Kaisipang Hudyo sa Deuteronomio 20:18: Guide for the Perplexed

The infidels, however, though believing in the existence of the Creator, attack the exclusive prerogative of God, namely, the service and worship which was commanded, in order that the belief of the people in His existence should be firmly established, in the words, “And you shall serve the Lord,” etc. (Exod. 23:25). By transferring that prerogative to other beings, they cause the people, who only notice the rites, without comprehending their meaning or the true character of the being which is worshipped, to renounce their belief in the existence of God. They were therefore punished with death: comp. “Thou shalt save alive nothing that breatheth” (Deut. 20:16). The object of this commandment, as is distinctly stated, is to extirpate that false opinion, in order that other men should not be corrupted by it any more: in the words of the Bible “that they teach you not,” etc. (ib. is). They are called “enemies,” “foes,” “adversaries”; by worshipping idols they are said to provoke God to jealousy, anger, and wrath.

You have, no doubt, noticed in the Torah how the commandment to annihilate the seven nations, and “to save alive nothing that breatheth” (Deut. 20:16) is followed immediately by the words, “That they teach you not to do after all their abominations, which they have done unto their gods: so should you sin against the Lord your God” (ib. 18); that is to say, you shall not think that this commandment implies an act of cruelty or of retaliation; it is an act demanded by the tendency of man to remove everything that might turn him away from the right path, and to clear away all obstacles in the road to perfection, that is, to the knowledge of God.