so that they cannot teach you to do all the detestable things they do for their gods, and so cause you to sin against the LORD your God.
They are here directed what method to take in dealing with the cities (these only are mentioned, Deu 20:10, but doubtless the armies in the field, and the nations they had occasion to deal with, are likewise intended) upon which they made war.
Commenting on Deuteronomy 20:10-20
That they teach you not to do after all their abominations,.... This is another reason why they were to be utterly destroyed, not only because of the abominations which they committed, but to prevent the Israelites being taught by them to do the same; wherefore, as before observed from Jarchi, such as became proselytes were suffered to live among them, because there was no danger...
Deu 20:15-18 It was in this way that Israel was to act with towns that were far off; but not with the towns of the Canaanites (“these nations”), which Jehovah gave them for an inheritance. In these no soul was to be left alive; but these nations were to be laid under the ban, i.e., altogether exterminated, that they might not teach the Israelites their abominations and sins (cf.
Commenting on Deuteronomy 20:15-18