Such evil commands are among the most difficult things for the religious to justify. But of course that doesn't prevent them from trying. Copan, along with such apologists as William Lane Craig, Norman Geisler, Thomas Howe, and probably many others, claim that this slaughter of children was in fact a good thing. Why? Well, according to Craig, since “God’s grace is extended to those who die in infancy or as small children, the death of these children was actually their salvation” – the idea being that otherwise they would be hell-bound. Geisler and Howe, in their Big Book of Bible Difficulties, even have the gall to describe the slaughter as “an act of God’s mercy” towards these helpless victims.
Now, atheists of course aren't going to agree – but that might be because we do not follow the dictates of an all-knowing and perfectly good being, and as a result have a screwed-up sense of right and wrong. If, however, one does believe that Yahweh is morally perfect, one must find some justification for the slaughter – and the only way such an action could be justified is if the children actually benefited from it. Anything less (such as Craig's alternative justification that God has the right to kill whomever he pleases) is – again for anyone who is decent and rational – wholly inadequate.
But even if one is convinced that the slaughter was good for the children, a problem remains. For if the fact that killing the conquered children at an early age was justified as a way of giving them a “get into heaven free” card, then why wouldn't the same thing go for many – in fact, most – children living today? After all, only some of us, according to Christian apologists, make it to heaven, so allowing anyone to reach an age at which they might believe in the wrong thing places their eternal soul at great risk.
Now, there are a few ways an apologist might attempt to differentiate the Biblical slaughter from other cases of infanticide. For one thing, God commanded the Biblical slaughter, but apparently isn't commanding anyone to go on a similar rampage today. But that doesn't really work, not if what justifies the Biblical slaughter is that it was a good thing for the children. Whether or not God commands it is in that case irrelevant.
Another thing one might say is that in the Ten Commandments, God specifically prohibits killing. However, the real meaning of that isn't “thou shall not kill,” but rather “thou shall not murder.” Killing can be justified, for instance, in self-defense, certain wars, and so on. And if what one is doing is a good thing, why wouldn't it be justified? That is what the apologists usually claim in the case of the Biblical slaughter: there is something that makes those killings good rather than bad; otherwise God would be commanding something wrong (and would be violating his own injunction against murder).
The best way for the apologist to avoid the horrific conclusion that infanticide ought to be practiced is for him to point out that there is a better alternative. It is preferable to send souls to heaven even earlier. An abortion, especially one in the first trimester, is certainly easier to accept than infanticide, and is in several respects better. (There is much less emotional attachment on the part of the parents, the fetus doesn't have any desires or beliefs yet, it is easier to convince the nonreligious to have one, it isn't against the law, and so on.) And yet, according to the view under discussion, it too sends a soul straight to heaven.
Those who approve of the Biblical slaughter of children should therefore regard abortion very positively. It has all the good benefits and much less in the way of bad ones. In fact, given that presumably it is a good thing to send as many souls to heaven as possible, Christian conservatives should be encouraging women to get pregnant for the sole purpose of aborting their fetuses – and doing this as often as they can! They should stop protesting abortion clinics and instead hand out fliers informing women of the religious benefits associated with the practice, and encouraging them to do the godly thing.
Or maybe they should just reconsider Old Testament morality instead...