In response to the Quora question “I feel like a lot of evil actions in the world have supporters who justify them (like Nazis). Can you come up with some convincing ways in which some of the most evil actions in the world could be justified?” David Pearce writes:

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“Tout comprendre, c’est tout pardonner.”
(Leo Tolstoy, War and Peace)

“Despite everything, I believe that people are really good at heart.”
(Anne Frank)

The risk of devising justifications of the worst forms of human behaviour is there are people gullible enough to believe them. It’s not as though anti-Semitism died with the Third Reich. Even offering dispassionate causal explanation can sometimes be harmful. So devil’s advocacy is an intellectual exercise to be used sparingly.

That said, the historical record suggests that human societies don’t collectively set out to do evil. Rather, primitive human emotions get entangled with factually mistaken beliefs and ill-conceived metaphysics with ethically catastrophic consequences. Thus the Nazis seriously believed in the existence of an international Jewish conspiracy against the noble Aryan race. Hitler, so shrewd in many respects, credulously swallowed The Protocols of the Elders of Zion. And as his last testament disclosed, obliquely, Hitler believed that the gas chambers were a “more humane means” than the terrible fate befalling the German Volk. Many Nazis (Himmler, Höss, Stangl, and maybe even Eichmann) believed that they were acting from a sense of duty – a great burden stoically borne. And such lessons can be generalised across history. If you believed, like the Inquisition, that torturing heretics was the only way to save their souls from eternal damnation in Hell, would you have the moral courage to do likewise? If you believed that the world would be destroyed by the gods unless you practised mass human sacrifice, would you participate? [No, in my case, albeit for unorthodox reasons.]

In a secular context today, there exist upstanding citizens who would like future civilisation to run “ancestor simulations”. Ancestor simulations would create inconceivably more suffering than any crime perpetrated by the worst sadist or deluded ideologue in history – at least if the computational-functional theory of consciousness assumed by their proponents is correct. If I were to pitch a message to life-lovers aimed at justifying such a monstrous project, as you request, then I guess I’d spin some yarn about how marvellous it would be to recreate past wonders and see grandpa again.
And so forth.

What about the actions of individuals, as distinct from whole societies? Not all depraved human behaviour stems from false metaphysics or confused ideology. The grosser forms of human unpleasantness often stem just from our unreflectively acting out baser appetites (cf. Hamiltonian spite). Consider the neuroscience of perception. Sentient beings don’t collectively perceive a shared public world. Each of us runs an egocentric world-simulation populated by zombies (sic). We each inhabit warped virtual worlds centered on a different body-image, situated within a vast reality whose existence can be theoretically inferred. Or so science says. Most people are still perceptual naïve realists. They aren’t metaphysicians, or moral philosophers, or students of the neuroscience of perception. Understandably, most people trust the evidence of their own eyes and the wisdom of their innermost feelings, over abstract theory. What “feels right” is shaped by natural selection. And what “feels right” within one’s egocentric virtual world is often callous and sometimes atrocious. Natural selection is amoral. We are all slaves to the pleasure-pain axis, however heavy the layers of disguise. Thanks to evolution, our emotions are “encephalised” in grotesque ways. Even the most ghastly behaviour can be made to seem natural –like Darwinian life itself.

Are there some forms of human behaviour so appalling that I’d find it hard to play devil’s advocate in their mitigation – even as an intellectual exercise?

Well, perhaps consider, say, the most reviled hate-figures in our society – even more reviled than murderers or terrorists. Most sexually active paedophiles don’t set out to harm children: quite the opposite, harm is typically just the tragic by-product of a sexual orientation they didn’t choose. Posthumans may reckon that all Darwinian relationships are toxic. Of course, not all monstrous human behavior stems from wellsprings as deep as sexual orientation. Thus humans aren’t obligate carnivores. Most (though not all) contemporary meat eaters, if pressed, will acknowledge in the abstract that a pig is as sentient and sapient as a prelinguistic human toddler. And no contemporary meat eaters seriously believe that their victims have committed a crime (cf. Animal trial – Wikipedia). Yet if questioned why they cause such terrible suffering to the innocent, and why they pay for a hamburger rather than a veggieburger, a meat eater will come up with perhaps the lamest justification for human depravity ever invented:

“But I like the taste!”

Such is the banality of evil.