I think physically the most efficient would just be to basically strangle people to death. It's really not physically that demanding to strangle someone to death.
However, it is extremely demanding mentally. Which is why the "most efficient" is usually the "most detached" one.
It should be a tip to these people that they disgust themselves while doing what they claim to think is right. But it fucking isn't. And I don't know why.
I think you’re right. It also wouldn’t be unfair to say that, if you’re in a european/western world country that is likely going to be mostly white and also happens to have taken a big part in WW2, that of course it will be a big part of your history. For the US, it’s our history in Europe, but we don’t seem to claim or learn much about European history elsewhere, like Leopold or Churchill’s Bengal Famine.
I think many people are familiar with at least a few mass killers. Mao, Stalin, Pol Pot, maybe the Red Terror, Christopher Columbus, you could even count Khan as one of the biggest, but we might view that as a bloody past and the result of Khan’s conquest of other peoples rather than a internal conflict killing millions of a specific group. Columbus’ mass murder was partly out of ignorance when the European diseases spread in the New World, but Europeans didn’t have too much of a problem with killing natives otherwise.
Hitler, OTOH, as you said, turned mass murder into a pointless, cold, indiscriminate killing machine in a way that no other ruler did. People were warehoused until fed to the machine. The Soviets/Russians and Pol Pot (influenced heavily by Stalin) are a pretty close second, they rounded up people and shipped them off to gulags or prisons and worked them to death if they weren’t just outright killed.