Brutality was a way of the times. Look at the Roman brutality in the arenas feeding Christians alive to the animals. And Roman historians have the audacity to accuse the Celtic invaders of being "barbarians"? I suspect most of the references calling one nation or another "brutal" is all in the eye of the author.
God told the Judeaens to go into captivity with the Babylonians and they would be OK, he didn't promise the same with the Northern Tribes. I believe there was a difference in the level of brutality.