I agree that there's a particularly European tradition of non-total war where maybe you burn the crops and lay seige but let the peasants live because hopefully they'll eventually be your peasants, but the era was more than willing to utterly obliterate schismatics and heretics, and it's pretty obvious that various civilizations have fallen and been obliterated by conquerors of one sort or another throughout history, maybe not as a common norm, but not infrequently. Look at 1 Samuel 15:3 for example, not that I have a good sense of actual historicity of it.