Let me give you some true history behind the story. The people in Canaan were given 400 years to repent from their wicked ways. They sacrificed their children on the arms of the false God Molech (like we do today to our unborn) as a fertility rite. God ordered all of them to be slaughtered because they were wicked. He gave them 400 years to repent and they did not. HOw much time should God give them? a million years? Who are you to judge the perfect judge? You are a mere ignorant mortal. Furthermore, the children had to be killed because they would grow up to be just like their wicked parents, as the apple never falls far from the tree (if you need a modern example, look at the palestinians who train 4-year-olds to be murderous martyrs). In order to cleanse the land and protect the Jews from being polluted by their wickedness, they had to be erased.
Oh, that makes sense. Aborting babies is ok since many of them will grow up to be wicked anyway. I'm sure some of those mudered Columbine students would have grown up to be wicked, so it was ok to kill them too. Every adult will be wicked to some degree, so let's kill all those babies right now, eh?