I find the medical experiments performed by Japanese and german doctors during ww2 to be more frightening because they where done in the name of medical science by "modern" humans completely capable of emphasizing with their victims.
The great lesson of the Nazis and the Milgram experiments is that all/most humans are capable of great evil when it is socially normalized.
To think that humans do these things out of hate and dehumanization puts the cart before the horse. Rather, there are cultural drives that incentivize certain behaviors, leading people then suppress and rationalize away their empathy in order to engage in those behaviors and reap the direct or indirect social benefits.
The most common pattern is devastatingly simple: hating an out-group is a powerful way to create an in-group bond.
