Numbers 31:15 - Ellicott's Commentary On The Whole Bible

Have ye saved all the women alive? — It was the women, as is expressed in the following verse, who had been the cause, at the instigation of Balaam, of the apostacy of the Israelites; and consequently the command to “avenge the Lord of Midiani” implied the punishment of those who had been the instruments employed in the seduction of the Israelites.