Because Christians treat the Jewish scriptures as holy, they must contend with the morality of behavior depicted in those scriptures — both the behavior of those characters held up as exemplary and the behavior of Yahweh. How Christians deal with the issues raised by that behavior can tell us a lot about Christianity and Christians.
It's no surprise that Christians in the past either ignored the genocidal stories like those in the Book of Joshua (because they couldn't read the stories themselves) or accepted them as normal (because their own society was so violent). This began to change in the Enlightenment, though, as scholars and philosophers began to subject their own religion to more critical scrutiny.
One theological shift was especially important: whereas in the past Christian theologians assumed that whatever Yahweh did was good because Yahweh did it, during the Enlightenment they started to assume instead that Yahweh did or ordered things because they were good. This allowed them to evaluate the morality of actions and ordered attributed to Yahweh.
The importance of this shift should not be underestimated. The previous approach was ultimately passive because it required a person to accept as legitimate, just, good, and moral, whatever was attributed to God — no questioning, doubt, or skepticism was permitted. The Enlightenment approach, in contrast, required a more active engagement with both the text and one's own moral reasoning. It required the Christian to not only make a judgment about the actions and commands attributed to God, but take responsibility for that judgment.
As a consequence, many Enlightenment thinkers concluded that the stories of genocide were patently immoral. This contributed to some leaving Christianity entirely because they couldn't remain part of a religion which worshipped such a barbaric deity. Others concluded that the stories were simply a product of their times — that the ancient Israelites lived in a violent age, were as violent as other societies around them, and naturally believed in a god that would command them to do horrible, violent things.
As John Rogerson writes in "The Old Testament: Historical Study and New Roles," in Companion Encyclopedia of Theology:
"The Old Testament was not, therefore, a collection of examples of pious living worthy of imitation by Christians; it contained stories of Israelites who lived in barbaric times when human life was valued cheaply, and when belief in God was sufficiently primitive for people to believe that he could legitimately command immoral acts."
These realizations did not entirely end all Christian use of genocidal stories in their theological systems, though. One major reason is that so many Christians have refused to accept the premise that their god only does or commands things which are good. Instead, they hold to the older view that whatever their god does or commands is, by definition, good. Combined with reading the texts as literal, factual history they conclude that the genocidal destruction of the Canaanites was necessarily a good act.
As a consequence, more than a little bit of time and effort is invested into trying to get people today to accept that genocide is good when Yahweh orders it. Modernity is in large part a product of the Enlightenment, which means that the sorts of moral reasoning that characterized the Enlightenment are now taken for granted. People aren't as willing to just accept without question the genocide can be good for any reason, even this one, so apologists struggle to find other rationalizations.
William Lane Craig, for example, argues not only that Yahweh was perfectly justified in ordering the Israelites to slaughter the Canaanites, but that any such orders that might come today would be equally justified. Indeed, he argues that we humans have a moral obligation to commit genocide whenever and against whomever Yahweh commands:
The command to kill all the Canaanite peoples is jarring precisely because it seems so at odds with the portrait of Yahweh, Israel's God, which is painted in the Hebrew Scriptures.
...According to the version of divine command ethics which I've defended, our moral duties are constituted by the commands of a holy and loving God. Since God doesn't issue commands to Himself, He has no moral duties to fulfill.
He is certainly not subject to the same moral obligations and prohibitions that we are.
This is a direct defense of the pre-Enlightenment idea that whatever Yahweh commands is automatically good, but Craig still felt it necessary to argue that the command was somehow good for independent reasons as well:
By the time of their destruction, Canaanite culture was, in fact, debauched and cruel, embracing such practices as ritual prostitution and even child sacrifice. ...So whom does God wrong in commanding the destruction of the Canaanites? Not the Canaanite adults, for they were corrupt and deserving of judgement. Not the children, for they inherit eternal life.
So who is wronged? Ironically, I think the most difficult part of this whole debate is the apparent wrong done to the Israeli soldiers themselves. Can you imagine what it would be like to have to break into some house and kill a terrified woman and her children?
The brutalizing effect on these Israeli soldiers is disturbing.
What William Lane Craig is describing here is a completely amoral being — a being that cannot even conceive of morality, much less act morally and exist in any sort of moral relationship. It's little wonder, then, that it would be described as massacring large numbers of people without second thought or a tiny twinge of the conscience. It has no conscience. No empathy. No moral sense whatsoever.