Villains who have abandoned Judaeo-Christian morality and gone Nietzschean are interesting, both in their view of the strong or superior or X will rule.
Villains who are immoral but shown in some flashback to have been abused, thus explaining and excusing it, lose their appeal. And that kind of excuse is an insult to everyone tortured and abused but didn't become a mass murderer / child molester.
Villains who are simply crazy but geniuses are intriguing, but when writers in a sequel say "oh, it is PTSD" or "he was abused by the system", it again undermines the appeal of the character, the open curiosity of what could make them that way, and is an insult to all who underwent trauma and didn't become violent predators.
Heroes tend to be interesting but follow standard tropes, and the modern anti-hero where he is just better than the bad guys is overdone.