I read a story once in Reader's Digest many years ago about this detective who was an FBI profiler. He stated that for a long time he didn't believe in good or evil. However, his opinion changed after he interviewed a couple of guys who would film their victims while they tortured them which I won't mention details but suffice to say it made crucifixion look merciful. They were found to be mentally competent for trial & got the convicted but it was interesting because he was absolutely baffled & had no reference point or explanation for their behavior as they weren't found to be mentally incompetent/insane & there was no familial history, or any of the other typical patterns, etc.... So he concluded that evil does exist as he saw it. Anyhow, doesn't change anything for any of us as we are going to believe what we choose to believe but that experience definitely impacted his own life & beliefs so much so he wrote a book about it that was excerpted & included in Reader's Digest. I'm sorry I can't remember more so you could read it yourself as I know you actually like & take the time to read but it has been a very long time ago. It just stuck with me because the details (of which he only shared a portion) were simply some of the most depraved acts I've ever personally seen put to print.
People do horrible unspeakable acts to other people every day & at least some are quite mentally competent when they do so. While I actually have some experience with mental illness (yes, I can hear the jokes now...

& can attest it absolutely can & does impact people, one's moral compass isn't always impacted by mental illness, it depends upon the specific characteristics. So it's not a panacea for all the ills & evil of mankind but is definitely a contributor. However, whether we believe that good or evil exist is a fundamental value attribute of our belief system I think. My only comment would be as I've said before, if one does not accept the existence of evil, one must not accept the existence of good either. So then, actions are merely choice & consequence in physical reality with no other attributes or characteristics of interest beyond the physical ramifications. However, like that article I posted many months ago written by the formerly athiest/humanist professor of philosophy who eventually saw the inherent hypocrisy of his humanistic beliefs, he became a nihilist. In other words, either both edges exist (evil implies good & vice-versa) or the sword doesn't exist altogether (there is no morality). While I obviously don't agree with him, I respected the fact that he realized the intrinsic lopsidedness & hypocrisy of his former position. I'm not putting you into any category btw, just making my own comment on the topic.
best regards,
Pedal2Metal