Numbers 31:7-18 - Moses was angry with the officers

31:7-12

So they attacked Midian and there was much killing. Five kings of Midian were killed. Balaam was also killed. He was the seer who Balak had sought to employ to curse Israel, but ended up blessing them instead, much to Balak’s annoyance. As noted there, Balaam is a puzzling figure, but the Bible takes a very dim view of him. It seems that the women and children were spared, and were taken as captives. There was much killing and plundering.

31:13-18

Moses was very angry with the army for sparing the women and children. So Moses ordered them to kill all the boys, and all the women who were not virgins. Only the virgins were to be spared. This seems incredibly harsh, indeed wanton violence. We are not told that the Lord approved or disapproved of this. Moreover, Moses’s word that they could keep the virgins doesn’t exactly seem to be an act of moral rectitude! 

We do learn why Balaam is regarded the way he is in the Bible, for it was he who told the Midianites how to defeat the Israelites. Namely, by using the women to entice the Israelite men.