In terms of sheer body count, the Nazis were pikers.

Stalin killed between 20 and 40 million by starvation.

Mao's "Great leap forward" or whatever you care to call his slaughter, killed between 60 and 100 million Chinese, and no one is really sure.

That having been said.......

I've been to a concentration camp site. While in Germany for an exercise, I visited Bergen-Belsen, a site where Jews and Russians were killed wholesale.. It is without question the eeriest place I have ever been. While there, I was stunned by the absolute silence. The only sound was the wind in the trees. No birds made a sound, nothing. It was as if nature had abandoned the site with the exception of plants.

No one on the grounds was saying anything. Just walking through, aghast at the graves, which simply had the number of corpses the allies THOUGHT were buried in them. 20,000, 40,000. A monument to human stupidity and cruelty.