20:11 tributaries. Subject peoples forced to “contribute” labor.
20:16 save alive nothing. This commandment, repeated through Moses and Joshua in various ways and times during the exodus and conquest, has been the object of tremendous criticism by enemies of Biblical theism. Such critics have charged God with sadistic cruelty. The Lord, of course, does not need to defend His actions. Whatever He does is right, by definition. There is a time coming, in fact, when all who have rebelled against Him, rejecting His righteousness and His love (as had these Canaanites) will be “punished with everlasting destruction from the presence of the Lord” (II Thessalonians 1:9).
20:18 That they teach you not. This was the main reason for God’s sweeping judgment on the Canaanites. In fact, when the Israelites failed to carry out God’s command, they themselves were led into apostasy and finally into exile. It would have been better if these hopelessly apostate tribes could have been prevented from spreading their utter moral corruption to future generations. The immorality and cruelty of the Canaanite tribes has been fully confirmed by various artifacts and inscriptions found by archaeologists. Regarding the children who would be too young to choose right or wrong or to understand about God, we can assume such were safe in virtue of the future redemptive work of Christ.