Why did God condone such terrible violence in the Old Testament?

That's a great question. Seriously. It's a paradox, almost a koan. These are unanswerable questions, the contemplation of which can lead to all kinds of wonderful things. And if you think of ALL the sacred literatures of humanity and polemics (philosophical questions) they pose, rather than thinking they're simply supposed to provide pat and simple answers, you can find a lifetime's worth of fruitful intellectual pursuits.