John Gill Commentary Deuteronomy 2:34

"And we took all his cities at that time, and utterly destroyed every inhabited city, with the women and the little ones; we left none remaining:" — Deuteronomy 2:34 (ASV)

And we took all his cities at that time
As Heshbon, and others mentioned in (Numbers 21:25Numbers 21:30Numbers 21:32) (Numbers 32:3Numbers 32:34–38) ,

and utterly destroyed the men, and the women, and the little ones
of every city, we left none to remain ;
for the Amorites were one of the seven nations who were devoted to destruction, the measure of whose iniquity was now full, and therefore vengeance was taken.