Adam Clarke on Numbers 31:17

Kill every male among the little ones - For this action I account simply on the principle that God, who is the author and supporter of life, has a right to dispose of it when and how he thinks proper; and the Judge of all the earth can do nothing but what is right. Of the women killed on this occasion it may be safely said, their lives were forfeited by their personal transgressions; and yet even in this case there can be little doubt that God showed mercy to their souls. The little ones were safely lodged; they were taken to heaven and saved from the evil to come.