They slew all the males That is, they slew those who were in the fight, and who did not save themselves by flight. As a nation they had forfeited their lives to the laws of God, and he, as judge of all the earth, had given command that the sentence of his laws should be executed on the guilty. Le Clerc properly observes, that there was this material difference between the wars of other nations and those of the Jews, that the former followed the bent of their own passions; whereas the Jews were only the ministers or executioners of divine justice, most evidently appointed to this work by God himself, as he manifested by a train of miracles wrought among and for them, such as had never been known on the earth before. So that no consequence can be deduced from their conduct in their wars, to warrant the like conduct in other people. Balaam also they slew He suffered justly, for being the wicked instrument of seducing the Israelites from their allegiance to Jehovah.
Related Commentaries of Numbers 31:7
Numbers 31:7-8
7 And they warred against the Midianites, as the LORD commanded Moses; and they slew all the males.
8 And they slew the kings of Midian, beside the rest of them that were slain; namely, Evi, and Rekem, and Zur, and Hur, and Reba, five kings of Midian: Balaam also the son of Beor they slew with the sword.