Numbers 31:17 - Joseph Benson’s Commentary on the Old and New Testaments

Kill every male among the little ones Which they were forbidden to do to other people, (Deuteronomy 20:14,) except the Canaanites, to whom this people had equalled themselves by their horrid crimes; and therefore it is not strange, nor unjust, that God, the supreme Lord of all men's lives, who, as he gives them, so may take them away when he pleaseth, did equal them in the punishment. Kill every woman, &c. Partly for punishment, as having, in general, either prostituted themselves to the Israelites, or some way been accessary in enticing them to idolatry, in which they were so confirmed that there was no hope of reclaiming them; and partly for prevention of the like mischief in future; for had they been saved alive, they would probably have continued to lead the Israelites into the sin of fornication, and have poisoned their minds by their superstitions. The female children were to be spared, because, being young, there was some hope they might be reformed from idolatry, and become proselytes to the true religion. These they might have as servants, or might marry them.