I think you’re partially right. If a certain ethical code is chosen simply because it is regarded as having a divine source, there would be something necessarily nihilistic in giving the said source a positive weighting. However, it would only deny rationality itself as having any intrinsic value: but if the super-intellectual divine has absolute, intrinsic value simply as a part of its definition, the ethical code deriving from it would as well, and certain actions would become intrinsically desirable.
My point here is that there are many people who regard themselves as abiding by material reductionism as an overall worldview, but who simultaneously admit that the core of morality is essentially non-rational (Ex. the “void” described in the 13 Virtues of Rationality), and this is not internally consistent. My criticism is that attempts to answer ethical questions from a purely rational standpoint usually yield incorrect conclusions, when questions concerning the nature and origin of that non-rational component are removed from the equation.
I’ll concede the use of the word genocide, since you’re right: substituting “killing a girl’s entire family in front of her and then enslaving her” sounds just as bad.
The accounts of wars recorded in the books of the Prophets and Writings often describe women and children being killed in war by surrounding nations, such as Babylonia, Persia and Assyria; it was, revoltingly, a common practice. The rule of war laid down in Deuteronomy 20:14 only allows the Jews to kill adult males in the course of war, and forbids the murder of women and children. The exceptions to this rule were the Canaanite nations and the Amalekites; wars against these nations made no exceptions for women and children. This is why Moses needed to give a specific order concerning the Midianites: the Israelite soldiers assumed that the women ought to be spared, in accordance with Deut. 20:14, and Moses basically informed them that the adult women who attempted to sexually engage with Jewish men in pagan rituals should be killed as any other enemy combatants. It should also be noted that Deut. 20:10 requires the Jews to always offer a peace settlement before laying siege or running into battle, including the Canaanites and Amalekites (Numbers 31 describes them doing this with the Midianites), and forbids the killing of any men, women or children if the peace offering is accepted. In the event the offer is turned down, it is still forbidden to surround the entire enemy camp, and anyone who wishes to flee must be allowed to flee unharmed.
I’d be lying if I were to claim to be entirely at peace with all this: I am not. But evidence for the assertion that the Torah views violence and war as okay, rather than something to be diminished gradually, still seems lacking. The hope for the eventual abolition of war, and for peace between nations, is repeated far too frequently and clearly in the Prophets for the assertion to hold, and the assertion clashes with how most Jews have historically felt about unnecessary violence, beyond their very early, formative period of their history.