That cruelty may best be explained as the result of educated, powerful humanity everywhere sincerely regretting all the small people being squashed by the big people, but also regarding it as the result of an inevitable "Law of Nature" --- just a regrettable part of Progress's ever upward march.
In other words, Scientism made them do it : 'it' being "being a bystander to schoolyard bullying and yet doing little or nothing".
Scientism made them do it
Scientism can be best defined as science, at least as half remembered by middle aged, educated, powerful people ---- based on memories of their High School science education decades earlier.
High School science, unfortunately has always been and will always be a simplified, glorified, triumphantist, Whig account of Science with all of its real world complexities and unsolved issues swept under the carpet.
And the governing axiom of that pre-war scientism, and to some extent the entire science of Modernity, was "Hard Reductionism".
This was the faith - as yet unsupported by sufficient evidence - that all of reality can be explained, predicted and altered/improved by understanding the few simple universal and eternal laws that explained the physical motion of the smallest basic units of reality : atoms.
They (and their biological equivalent, the basic cell) occupied the lower left hand corner of the upward arrow of progress : being small, unchangingly simple and incredibly ancient.
By contrast, at the top right hand corner of that arrow of progress, everything was very new, very big and and incredibly and dynamically complex.
A complex bigness, but based simply upon being assembled from modules made up of multiple modules from the level of complexity just below them. And so on and so on back down to the original basic atoms.
So Life began when a few atoms attached around a single carbon atom becoming the small basic molecules of organic chemistry which then became, in turn, parts of much bigger biological molecules which became part of cells which became ever bigger multi-celled beings.
If re-defined as Soft Reductionism, the belief that much in reality can be explained this way, most of us still hold this view.
But in humanity's postwar view of science and in the scientists' Post-Modernity Science, we have decidedly rejected Hard Reductionism, explicitly or implicitly.
Scientists explicitly no longer see reality as a linear arrow ever upwards , but talk instead of non linear systems, non equilibrium physics, complexity science, chaos theory.
Basically they mean that after creating a modules of say 1000 atoms, it simply fails to display the known behavior of one atom multiplied 1000 times over - its actions are novel but unpredicted.
Further, even a handful of tiny different modules interact in extremely unpredictable fashion (unpredictable given the limited amount of world resources we can devote to computing) - let alone much bigger systems of interacting modules - such as the weather or the stock market.
Scientists say such things explicitly - we mere civilians tend to to more feel this sort of reasoning in our bones - believing less and less big things will turn out anything like the way experts, professionals and the powerful say they will.
So, back to WWII and its cruelties.
In a world believed to be totally predictable, things can be said to happen totally inevitably, with us unable to change them, even if we wanted to.
This belief is called Determinism, a higher level axiom in educated humanity's thought system, circa WWII.
It followed upon Reductionism, and it meant that while we might want to (our moral sentiment) save the Indian tribes in Canada, science had proven that the laws of nature had determined that these small ancient and simple societies would inevitably be replaced by the bigger and more modern structures of western civilization.
Regrettable to be sure, but one simply can't stand in the way of the bulldozers of progress : the 1940s Mosaic law of Robert Moses.
The arrow of progress is noticeably titled at a 45 degree angle - hard to square, at first glance, with another axiom of modernity : Darwin's claim of vertical only inheritance .
Darwin - wrongly - claimed we only inherit genes from our parents and they from their parents - vertically and linearly right on down to the tiny cells that first began life on Earth (this being the biological version of Reductionism).
Darwin claimed that we never get genes from our uncles or from total strangers - total strangers like viruses etc.
What he never claimed ,but that people assumed he claimed, was that each smaller module of life was, in the medium term, gradually replaced entirely by a slightly larger module and so on and so.
So horses started out small and then over time mutated slowly into ever bigger horses while the smaller older horse versions all died away.
This is the way that the world's best natural museums (I am not making thus up - God Help us !) illustrated the arrow of progress in the world of horses.
But it wasn't true and Darwin had never said it would be - success for any species, new or old, was in seeing its offspring survive because they were well suited to the niche they lived in.
So, in fact, in cold climates, horses are small, stout and well covered in fur - in deserts they are tall and thin.
What the arrow of progress really measured
Humanity's prewar arrow of 'evolutionary' progress, unwittingly I believed, was really based on a scale that measured only the progress of human type book learning and record keeping.
On this scale, yes, it seemed the first (the biggest, newest) were always first and the last (the smallest,oldest) were always last.
But evolution should really be measured by survival success, full stop.
Here the record is more clouded for big creatures like humans.
Microbes, thanks to the ability to exchange genes between themselves operate more as a single super organism, somewhat the way we humans are made up of trillions of co-operating cells.
Yet, on the surface, they seem so weak : a usually immobile tiny sac of mostly water : the smallest, oldest and weakest form of life.
But in terms of survival, they are the champions bar none.
Post 1945 : we realize this world is really made for microbes, not humans
The microbes as a collectivity have existed for 4 billion years when most single species, like humanity, survive for a million years at best.
They live everywhere imaginable on Earth - extremities of cold, heat, drought, acidity, starvation, radiation they toss off with ease.
In numbers of different individuals (because yes, like all forms of life and contrary to the tenets of reductionism, each individual of life ever born has always been uniquely different in subtle but important ways), they far outnumber all the rest of life put together.
They may represent the largest mass/weight of life in terms of biomass, though the term biomass is very hard to define (do dead tree trunks count ?)
These small and the last, may survive when the first and the big die, precisely because they are small - when niches get small and thin - only the small and thin get enough to eat to survive and reproduce.
And because, again contrary to Hard Reductionism, they are small but dynamically complex .
(Just as modernity's scientists soon discovered of the not-indivisible after all atoms. For each is made up instead of a complex seething soup of matters dark and anti, of spin and top and color. So much so that if anyone claims they truly understands the sub-atomic world, they're lying.)
Simply put, if mutations are needed if a species is to survive rapidly changing conditions, a small being that produces a new generation every twenty minutes instead of every twenty years that big beings need to reproduce, works its evolutionary magic more than 500,000 times faster.
Particularly when microbes are not biological racists like humanity circa 1940, instead being perfectly willing to take genes, horizontally, from anyone and anything.
Open commensality versus closed racism
The openness of commensality rather than the exclusivity of racism so that their gene pool gets ever deeper and they don't try to constantly drain it via eugenic murder.
In terms of reading a newspaper, yes the first (educated humans) are first and the last (bacteria) are last.
But in terms of making penicillin, WWII discovered - to its horror and surprise - the last ( the tiny slimy penicillium fungi) shall be first and the first (the world's best chemists) shall be last.
In terms of speed, neither the last (rare bacteria with limbs) nor the first ( obese urban university employed humans) are particularly fast in terms of body lengths travelled per second - that goes to the medium (cheetah and such).
Today instead of a single simple scale of worthiness with the new big and complex at the top inevitably, we accept that all forms of life excel at some things and do poorly in others.
We no longer believe that some life - inevitably, by laws of nature - must be life unworthy of life and so can be legitimately burned up in smoke at Auschwitz....