Namely, all whom they took in that war, or all who lived in those parts; for it is probable (and was then very usual) some colonies of them were sent forth to remoter places, which therefore had no hand either in their former sin, or ill this present ruin, of whom we read after this, Jud 6. And herein they did according to God's own order concerning such people, Deuteronomy 20:13; only their fault was, that they did not consider the special reason and great obligation which they had to involve the women in the destruction, for which reason Moses blames them afterward, Numbers 31:15,16.
Related Commentaries of Numbers 31:7
Numbers 31:7
7 And they warred against the Midianites, as the LORD commanded Moses; and they slew all the males.