Why did God command for ALL of the Canaanites to be killed - including women, children, and the aged?

This is a question that has puzzled and plagued many a Christian.  Some atheists plant their flag here, the hill to die on, and claim that our God is not a God of Love or mercy.  Instead, they paint the picture of a "god" who is nothing more than a bloodthirsty monster. Yet, how does a Christian answer this protest?  What do we do about these passages?