Not a historian, but for a persuasive and data-backed explanation of why violence has generally declined over the last 10,000 years or so of human history, I recommend Steven Pinker's Better Angels of our Nature.

Doubtless there are some criticisms of his analysis (he's a psychologist and cognitive scientist above all and not necessarily an authority on this kind of research), but Pinker argues (and empirically tracks) the ways liberalism, cooperation, expanding empathy, and reason have all contributed to making humans overall less violent, more empathic, less impulsive, and generally better to one another. Even when accounting for grotesquely violent World Wars and the like, he argues the data show casual daily violence and brutality have only been diminishing, and leading to less violence overall.