Why did God command Israel to destroy the Hittites, Amorites, Canaanites, Perizzites, Hivites, and Jebusites?
God commanded Israel to utterly destroy those nations so they would not teach Israel their abominations—the idolatrous and pagan worship practices they performed for their gods—and thereby cause Israel to sin against the Lord. The command reflected God's holy, righteous, just, and fair character, and He told Israel not to spare one.
Les teaches that Deuteronomy 20:17 records God's command to utterly destroy the Hittites, Amorites, Canaanites, Perizzites, Hivites, and Jebusites, and not to spare a one. The next verse gives the reason: lest they teach Israel to do all their abominations, which they had done unto their gods, meaning their idols and pagan worship, and so should Israel sin against the Lord their God. Les reminds us that what was an abomination to God back then is an abomination to God today. He then points to Solomon, who married 900 wives out of all the various pagan cultures and, in order to satisfy the demands of those pagan wives, was led into their practices.
But thou shalt utterly destroy them; namely, the Hittites, and the Amorites, the Canaanites, and the Perizzites, the Hivites, and the Jebusites; as the LORD thy God hath commanded thee: