Christians often feel squeamish about Old Testament passages where God commands the complete destruction of cities: men, women, and children. But this is because we have often imbibed a sentimentalism that is not really Christian, and it has had disastrous effects on the Church.
The central point that must be driven into our hearts is the supreme holiness and righteousness of God. When God commanded the complete annihilation of people in the Old Testament, we are required to say Amen. He is God, and we are not. He knows all things. He made all things. He kills and makes alive. Blessed be the name of the Lord.