The medieval era was much less violent than either the ancient or early modern era, in part due to religious laws. So please for once stop using medieval as an insult and admit Singer shouldn’t have done so.
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I think you can get into an interesting argument as to if that has more to do with restrictions on violence against co-religionists or weaker state capacity, though both seem to play a role. It is striking that when the norms are broken, it tends to be in religious conflict.
Though, for the sake of caveating my own argument, the Hundreds Years War is the rare instance of a very destructive conflict in the Middle Ages (albeit on the door-step of the early modern) which is not a religious conflict. So they do happen, just less often.