Okay. So what you are saying is that Mao and Pol Pat didn't actually care about atheism. So their motivations were not rooted in the belief 'Gods do not exist'. Given that we can probably argue about their motivations forever, I'll concede that as a fair point.
But regardless of their motivation, what was the justification for their actions? What reason did they give to justify the deaths?
Well, like any sociopath or psychopath? They simply didn't bother.
Look at the sociopath-in-chief currently residing in the White House: Does he ever justify what he says or does?
No. He does not even try.
He preaches empty platitudes, such as "make American great" and the like.
So, too, Mao and Pot (Stalin, Hitler and the rest) do not bother to justify what they do--
in fact, I seriously doubt it even enters any of their minds, that justification is necessary!
You and I (and most people) are Empathetic, and therefore,
not sociopaths. Thus, anytime we do something Extreme? We feel it needful to justify doing such a thing.
And even then we all too often suffer Post Traumatic Stress, for the aftermath!
These horrific people that we are speaking of? Crave Power. It feeds into their massively overblown Egos.
They do whatever they want, Because They Can, and Because They Think They Deserve To.
When they gain Ultimate Power, as a Head Of State? They become monsters....
If it were not for the very Clever and Fore-thinking people who wrote the US Constitution? Imagine how much of a rampage trumpEgo would be doing
Right Now!
Under Mao and Pot? There were no such limitations-- they has as close to absolute power as it was possible to have, and not be swallowed up by it.
As happened so often in History: Look at the principles in the French Revolution--- they were each, eventually removed from power by the very Engines they created.
I expect that is what happens most of the time-- History was simply re-written to seem less brutal, by the successors...
