John Calvin Commentary Deuteronomy 7:16

"And thou shalt consume all the peoples that Jehovah thy God shall deliver unto thee; thine eye shall not pity them: neither shalt thou serve their gods; for that will be a snare unto thee." — Deuteronomy 7:16 (ASV)

And you shall consume all the people. It is plain from the second part of the verse why He commands the people of Canaan to be destroyed, when He forbids their gods to be worshipped. This precept, therefore, corresponds with the others, where He in like manner dooms these nations to utter destruction. I now pass over what I have explained elsewhere, that is, that the vengeance which God exercised against these obstinate and ten-times lost people cannot be ascribed to cruelty. For since 400 years ago it had been said to Abraham that their iniquity was not yet full, they could not be treated with severity equal to their deserts, when they had so licentiously and wickedly abused God’s long-suffering.

But we must take notice of God’s design in so particularly instructing the Israelites to utterly destroy whatever should be found there. For besides His having already doomed them all to the destruction they merited, He also intended that the land in which His name was to be invoked be purged from all pollutions. Now, if any of the old inhabitants had survived, they would soon have endeavored to revive their corruptions; and since the Israelites were otherwise already too inclined to superstition, they would easily have been attracted to the worship of idols.

This, then, is the reason why God forbids them to show these people any humanity or clemency, as I have reminded you is clear from the context; for these things are connected—that they should not spare the nations nor worship their gods. The reason that is added, for it will be a snare or stumblingblock to you, must be extended to the whole context—namely, that it would be fatal to the Jews if they should spare the nations that would allure them to impiety.