Deuteronomy 20:18 - Joseph Benson’s Commentary on the Old and New Testaments

That they teach you not to do after all their abominations Here is the great reason for the aforesaid severe execution; they were most abominable idolaters, who offered their children to Moloch; they were magicians, wizards, necromancers, and guilty of all those abominations and filthy lusts mentioned Leviticus 18. So that God thought them not fit to live any longer upon the face of the earth; for had they been spared, after obstinately rejecting terms of peace, they would undoubtedly have sought to infect the Israelites with their filthy idolatry; and it was mercy to the human race in general not to suffer such a wicked, contagious generation to subsist. From the words here quoted, That they teach you not, &c., a Jewish writer justly observes, “If they repented and forsook their idolatry, the Israelites might let them live;” for then there was no such danger in sparing them. Accordingly Rahab, her father, mother, brethren, and all her kindred, were preserved alive, and so were the Hivites or Gibeonites, on condition of servitude, which they themselves offered, Joshua 9:11-15. See Joshua 11:11-20; Jeremiah 18:7-8.