Commentary for Numbers 31:15

?!החיים כל נקבה. Although it was a rule that in all wars not involving the seven Canaanite nations women, children, and livestock would be spared as we know from Deuteronomy 20,14, these soldiers should have at least killed the women whom they personally knew, seeing it had been the women who had caused the death of 24.000 Israelites by seducing them. [we have no evidence that any Midianite women seduced the Israelites with the exception of Cosbi, a Midianite princess. Numbers 25,1 reports only about sexual misconduct with the Moabite women. Ed.]

כל נקבה, “any female;”(among the prisoners taken); this is not the only time when the word כל, does not mean: “each” or “all;” we have other examples such as in Exodus 20,10: לא תעשה כל מלאכה, ”do not perform any menial work;” or Exodus 22,21: כל אלמנה ויתום, “any widow or orphan.” There are quite a few more examples that could be cited.