Consider the Mongol empire. While the Mongol empire had some things in common with a modern state, squeamishness about killing was not one of them. There are cases where they decided a city needed to be destroyed and they took great pains to make sure that destruction was total. Every man, woman, and child they could find was put to the sword (or axe). Depopulating an entire city was a difficult task before carpet bombing became an option. Mongol soldiers were given quotas and expected to produce enough ears to show that they had met that quota. Punishments for not meeting that quota were harsh. Ears were put into sacks and carted off in wagons to be counted. Even dogs, cats and chickens were killed in some cases. There are recorded instances of Mongol armies leaving towns after doing this and then deliberately returning a week or two later, just to make sure they got anyone who had managed to hide in a basement or who was out of town when they were first there.
Consider the Romans. After the third Punic war Carthage's population was sold into slavery en masse and the city burned for 17 days. The earth was salted. There was no fourth Punic war because there were no Carthaginians left alive and free. Let's not even talk about the Assyrians!
Many states throughout history have committed genocide against enemies and many others have persecuted populations within their own borders. Concentration camps are only necessary now because most people won't stand for such atrocities, and states therefore feel compelled to carry them out in relative secrecy. Modern human society is gentler now than at any point in our past, although perhaps navel-gazing and self-accusatory articles like this are part of the reason why, no matter how uninformed they might be.