A lot of those so-call normal people were literal Nazi party members who became such voluntarily. In particular, camps were run by SS - the elite of believers.
Literally nothing about Germany proves your thesis. Germany proves that it is possible to create political system under which you reward cruelty and consequently people become cruel. It shows that when politicians promote violence, violence can happen. It shows that when you punish opposition, opposition will be helpless.
This only applies to Altreich citizens, who lived in Germany as of 1933. But the Holocaust involved a lot of people who weren't Altreich citizens.
Austria was only annexed in March 1938, and yet supplied its share of war criminals to the Nazi effort.
The Trawniki men, who guarded a lot of the concentration camps in the East, were Ukrainians who only came under German rule in summer 1941 and didn't even speak German. I would say that the influence of German propaganda on them was negligible compared to an Altreich citizen.
OP referred to supposed process in which people spontaneously become super cruel. The claim that "it is possible to make people cruel by authorities" is something completely different.
We also have the real-life Nazi Germany experience, where 1000's of previously-normal people did unspeakable things. And of course the Japanese behavior in China before that. You might call these "natural experiments."
With human social behavior it's just not possible to have a rigorously controlled experiment. Nor can you ever end up with rock-solid conclusions similar to those in physics. Even more recent behavioral economics experiments are full of artificial situations, like $10 "bets" because you can't use serious sums. All you can say about critiques of SPE is "compared to what?"