I think maybe Josh's response was a play on words, cutting in both directions.
The Nazis were National Socialist. One "form" of socialism, although not in line with how we think of socialism today. Yeah, they did elevate mass murder to a pretty high level.
But under Communism in the Soviet Union, the Union of Soviet Socialist Republics, the death toll was several times the genocide of the Nazis. The exact number will be argued endlessly, and never proven, but few doubt it was at least 3 times the Holocaust. Another "form" of socialism.
And there are other examples of Socialist and Communist regimes with exceptional state directed high murder rates, but those two are probably the top of the heap.
It is sometimes hard to discuss history without tinctures of politics.
T!