So, what were the building blocks that had to be in place to enable otherwise good German people to do horrible things?
The drive of National Socialists (Nazis) to capitalize on discontents
a) An open subculture of deviance (the Wiemar period)
b) A perceived national humiliation after TKO in WWI
c) A collapsed economy (post-WWI hyper-inflation in Germany, followed by global depression)
d) Intense conflict between Communists and Nazis for ideological dominance
e) A very intense and long propaganda drive to demonize Jews as a contaminating other
f) Traditions of authority and obedience in family, school, church, work, and society
g) Long-standing and strong antisemitism in Germany, Poland, Baltic area, Ukraine, and Russia
The French, Italians, British, Scandinavians, Hungarians, et al didn't have share the circumstances and history of Germany.
The atrocities and dark decades of the Soviet Union didn't spring from the same conditions as did the atrocities and dark decades in Germany.
Neither did the American Experience. We too perpetrated world-class atrocities (extensive slavery and genocide). These practices began when we were still part of the British Empire, which went on to produce a few of it's own atrocities and dark decades. But we can't blame the British for our persistence after independence. From the standpoint of blacks and native Americans, we never did sincerely cease and desist. That's why they are where they are in American society.
So, Mai Lai
may have been a relatively isolated event as far as American troops were concerned, but it didn't seize the public (at least as far as I can remember). The anti-war groups and others were properly appalled, but they (we) were pretty much appalled all the time anyway. The Vietnamese were effectively an "other" group. Outré. Not like us. They were important only because they were perceived as a domino piece that would lead to a wider more Communist Asia -- and of course, the cliché, "If we don't stop them there we will have to fight them on the coast of California".
The Anti-war people didn't know much about Vietnam either. They were the abstraction of "victims of U.S. militarism" -- not real people, for the most part.
What were the conditions which enabled Milgram and Zimbardo to coax American college students into behaving badly?
Reasonably well educated Americans (not just college graduates) tend to view science and scientists favorably. What scientists do (SCIENCE!) is a good thing, by its very nature.
Participating in actual scientific experiments has a positive status value. Plus, it is usually at least moderately -- and sometimes very -- interesting.
While we do value life, liberty, the pursuit of happiness, and the Republic for which it stands, we also tend to be ardent individualists with a strong (sometimes residual) streak of a Calvinist and Roman Catholic theology which considers us to be fallen and prone to sin. It's a fatalistic streak in a generally positive outlook.
College students almost never live in a "total institution" -- colleges are pretty porous socially -- but colleges are somewhat 'set aside', and students do tend to be away from their home and childhood community for the first time and may not behave quite the same in college as they would back in Peoria or Brooklyn or San Diego.
TO MAKE A LONG STORY SHORT, SS troops didn't guard Auschwitz for the same reasons American troops killed peasants at Mai Lai, and American college students didn't participate in Milgram's experiments for the same reasons that Germans calmly watched Jews being shipped off "to the east".
The lesson is that we can perform very bad acts when the situation is properly (even if not deliberately) set up.
Right now, almost certainly normal, good people--American ICE agents--are separating crying toddlers from horrified parents and keeping them separately, well out of sight of each other in custodial detention. They are doing this because it is part of a national policy which many Americans agree with (limiting immigration, especially illegal immigration across the border) and maybe feel that separating parents and children for a while will be sufficiently traumatic to discourage another attempt at entry, after they are deported.
Extreme political statements, intense media coverage over the last decade or two, economic dislocation, declining standard of living and declining income among working class people, immigrant waves moving around elsewhere in the world, etc.--all contributes to the ability of ordinary people to perform this separation of children and parents.