And they warred against the Midianites, as the LORD commanded Moses; and they slew all the males.

Wesley's Notes

31:7 All the males - Namely all who lived in those parts, for colonies of them, were sent forth to remoter places, which therefore had no hand either in their former sin, or in this present ruling. And herein they did according to God's own order concerning such people, Deu 20:13, only their fault was, that they did not consider the special reason which they had to involve the women in the destruction.

Jamieson-Fausset-Brown Bible Commentary

7. they slew all the males-This was in accordance with a divine order in all such cases (De 20:13). But the destruction appears to have been only partial-limited to those who were in the neighborhood of the Hebrew camp and who had been accomplices in the villainous plot of Baal-peor (Nu 25:1-3), while a large portion of the Midianites were absent on their pastoral wanderings or had saved themselves by flight. (Compare Jud 6:1).

Matthew Henry's Concise Commentary

31:7-12 The Israelites slew the Kings of Midian. They slew Balaam. God's overruling providence brought him thither, and their just vengeance found him. Had he himself rightly believed what he had said of the happy state of Israel, he would not have thus herded with the enemies of Israel. The Midianites' wicked wiles were Balaam's projects: it was just that he should perish with them, Ho 4:5. They took the women and children captives. They burnt their cities and castles, and returned to the camp.