John Gill Commentary Deuteronomy 20:16

"But of the cities of these peoples, that Jehovah thy God giveth thee for an inheritance, thou shalt save alive nothing that breatheth;" — Deuteronomy 20:16 (ASV)

But of the cities of those people, which the Lord your God does
give you for an inheritance
The cities of the seven nations, six of which are mentioned by name in the next verse:

you shall save alive nothing that breathes ;
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 of which they were reserved to this destruction, when the measure of their iniquities was full, such as idolatry, incest, witchcraft, soothsaying, necromancy see (Leviticus 18:3Leviticus 18:24Leviticus 18:25Leviticus 18:27) (20:2-23) (Deuteronomy 18:9–12) .