You have to keep in mind the differences in culture from when these books were written and the culture of modern times. Violence was much more prevalent and hard to control on a societal level in Antiquity.
Even between the Old and New Testaments there lays some five centuries of social and political change. The NT is markedly less promotional of violence than the OT, because they were crafted in entirely different contexts. The Old Testament is a descriptive narrative of the Jewish people's mythical history, not unlike the corpus of Greek myth; violence and barbarity were just a fact of life, and it seems to be describing moreso than condoning violence, and where it is condoning violence it is not being very much different than any other society at the time. The New Testament, on the other hand, functions more as a guidebook to civil disobedience fused with the heroic narrative of a particular figure, and as such seems to disavow most forms of violence as either pointless or just making it worse for those following its propositions. When we get to the Quran, we find a narrative much more along the lines of the Old Testament, having to do with the struggles of the Arabs to unite and expand, with elements similar to the New Testament in that it concerns the personal struggle of a specific person (Muhammad); it deals with violence, as with the OT, in a more descriptive manner than prescriptive.
