The two most recent books I have read are Antony Beevor's The Second World War, and Empire of the Summer Moon. These two books are radically different in scope, one covering the biggest and most intense conflict in human history, the other a series of tiny skirmishes fought over decades in a near-empty wilderness. And yet they have one thing in common: they make you aware, in no uncertain terms, of the absolutely extraordinary extent of "man's inhumanity to man".
In Beevor's book we read of SS men at Treblinka picking young children up by their ankles and swinging them down like mallets to dash their brains out on rocks, Red Army soldiers gang raping and murdering Russian women who were enslaved by Germans and who they had just "liberated" on their invasion of East Prussia, Japanese soldiers from different units abducting each other to cannibalise, Soviet officers who have done nothing wrong being summarily shot by their superiors to instil discipline into others, Allied bombers flattening Dresden just because it was the only city in Germany they hadn't flattened, the USAAF judging the success of its missions in Japan on how many innocent Japanese women and children it had burned to death, citizens of Leningrad abducting and killing their own neighbours' children to eat during the winter of 1941....and so on and so on.
In Gwynne's, we read of Comanche raiders gang raping and scalping pregnant women; slicing off rival tribe members' noses, arms and legs and then burning them alive; cutting off the bottoms of captives' feet and making them walk around for their own amusement; Texan militiamen shooting Cheyenne women and children who surrender to them and then mutilating their corpses in horrific ways; Texas rangers killing thousands of head of horses in cold blood to stop them falling into Comanche hands; Ute women bludgeoning helpless octogenarian Comanches to death with axes...and so on and so on.
It's enough to make one wonder: if this is the kind of thing human beings routinely do to one another in certain circumstances, what kind of stuff would orcs do? And let alone orcs - what would demons and devils do? What, indeed, does "evil" really even mean, given what we know about history?
This is the main reason why I lean increasingly towards humans as the key protagonists and antagonists in my campaign settings. Anything an orc can do, we can do better. And who needs a goblin when the thought of being tortured to death by a fellow human being is so much more plausible and disturbing?