What concentration camps reveal is that modern people are squeamish about killing. 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 th…
Methodical killing of any type at scale is very rare in the modern world. It's even rarer for it to to be conducted through camps; to my knowledge, only the Nazis ever did it (and their allies, the Croatian fascist Ustaše regime). Most mass killing simply consists of taking groups of people somewhere remote and shooting them. Concentration camps are what you do when you don't want to do that and are basically a cheap form of prison. Even in the Soviet Gulag, where the harsh conditions and threat of death were part of the punishment, the majority of prisoners survived.
Even where death is tacitly the expected outcome of a camp (rare), it's mostly done by slow starvation and overwork. I think that's interesting, given how inefficient that is. It provides a kind of mental "plausible deniability".