That they teach you not to do after all their abominations, which they have done unto their gods; so should ye sin against the Lord your God. (Deuteronomy 20:18)
This points to the supreme reason for the severe measures taken. Great wickedness coupled with the rejection of mercy demanded judgment (see chs. 7:26; 12:31). When “the iniquity of the Amorites” was full, judgment came (see Gen. 15:16; 1 Kings 21:26).