Charles Ellicott Commentary Numbers 31:7

"And they warred against Midian, as Jehovah commanded Moses; and they slew every male." — Numbers 31:7 (ASV)

They slew all the males. —The reference appears to be to those who were engaged in the war. The words do not refer to the entire male population, as is evident from Numbers 31:17; and it is probable that many of the Midianites who were not engaged in the war withdrew from the scene of conflict. The reference in this verse, unless the words are inserted proleptically, seems to be to all the adult males who fell into the hands of the Israelites during the war.