So, it finally clicked that while the average person does in fact broadly comprehend that people are neither good nor evil - they're good and bad, and have free will - they also can't understand why some people would fully commit themselves to completely awful causes or to being a terrible person throughout their entire lives. They can't really picture how this works, because they can't imagine themselves choosing to die on a hill of Being A Terrible Person.
This void in their comprehension is where the myth of the Ontologically Evil Person is very likely to come and settle in sooner or later, because it seems to finally provide an answer that makes sense of otherwise senseless cruelty and violence. Agonizing questions like "Why would my boyfriend spend so much energy on making me feel like shit and breaking me down?" "Why would this historical figure decided to kill all of these people?" and "Why would this guy go start a cult and murder everyone?" are finally given an answer, and the formerly-bewildered person finally has some peace of mind.
Because of this, the myth of the Ontologically Evil Person is incredibly hard to get out of people's minds once it takes root. For one thing, bad ideas are like bad habits; it doesn't really work to tell people to Just Stop With Them, because without something else to take its place? They're going to fall back on it.
And if somebody's been traumatized from abuse? The last thing they want to hear is that they're basically dehumanizing their abuser and that's not cool, because it feels to them like the other person is taking their abuser's side and telling them to get fucked. Even if this not what's happening, the survivor's brain is currently operating on fight/flight/fawn/freeze mode, and a brain operating fight/flight/freeze/fawn mode is keyed to making snap decisions to try and remove you from the danger as soon as possible, which means categorizing everything into black and white. This person couldn't care less about the history of eugenics right now; literally all they care about is being safe.
"Okay, so if the Ontologically Evil Person doesn't exist, how the hell do you explain those fuckers over there?" some of you are probably asking.
Here's the deal. Literally every human being alive can and will do terrible things if they're sufficiently scared and desperate. They're in no position to appreciate that nearly all asshole behavior can be explained by a lack of critical social and self-management skills, or by a lack of access to self-improvement (including being too traumatized to trust means of self-improvement).
People who are scared, insecure, and under high levels of stress will often cling to anything that makes them feel better, because they want to feel safe and secure and not in psychological and/or physical agony. (Stress does an absolute number on your body, too.)
Being reliant on a shitty behavior, belief system, or product for some measure of feeling secure and safe is how you get people saying things like "If I didn't act mean, everyone would just walk all over me!" or "I was really depressed before I found this, so if I gave it up I'm going to get depressed again, and I might hurt myself." (And there might be some truth to this one! This might indeed happen if they give it up cold turkey, and without finding an alternative!) It's how you get people conducting """scientific""" studies to """prove""" that their bigotry is totally justified and not at all irrational. ("Well of course these people are genetically inferior, they wouldn't be poor and disease-ridden if they weren't... what do you mean, systemic inequality and uneven healthcare access? No that's obviously fake and made up by More Bad People.")
People also act in unhealthy ways to deal with personal insecurities implanted by parents or society. You have people out there whose parents drummed it into their heads that second place was for worthless losers, or that no one would love them if they didn't look or act a certain way. You have people who absorbed the idea that acknowledging the basic humanity of shitty people means that they have to forgive them and personally help them get better and just suffer through the abuse in the meantime.
This is how people choose to die on the hill of Being A Terrible Person. They weren't ontologically evil. They were scared, and they thought they saw a fortress on the top of that hill that would keep them (and perhaps also their loved ones) safe.
