Deuteronomy 20:16

But of the cities of those people, which the Lord thy God doth give thee for an inheritance,.... The cities of the seven nations, six of which are mentioned by name in the next verse:

thou shalt save alive nothing that breatheth; the reason of this severity was because of their wickedness, the capital crimes and gross abominations they were guilty of, and for which they deserved to die; and on account whereof they were reserved to this destruction, when the measure of their iniquities was full, such as idolatry, incest, witchcraft, soothsaying, necromancy, c. see

Leviticus 18:3.