Watch Walt Disney's animated classic Sleeping Beauty today and the most shocking moment may well be when Maleficent, just prior to transforming herself into a dragon, openly invokes "all the powers of hell." That film was released in 1959, at a time when simply saying that a villain was evil was both a sufficient and satisfactory explanation for why she must be opposed and defeated.
Today villains need origin stories. Evil must have a psychological, sociological, or biological cause. Today, we reject the doctrine of original sin. Instead we preach that man is the measure of all things and is (at least individually) inherently good.
Yet in the last two centuries, while this secular doctrine has taken hold, we've been bombarded with daily reminders of how capable we are of doing works of great evil. We fall into the explaining by naming fallacy, calling perpetrators of such actions "ill." As though replacing the word "sinful" with "sick" somehow clears up the eternal mystery of how those born good became capable of doing so much bad.
Or we look for a trauma that turned the young and innocent into a warped vengeance seeker. The worse the traditional villains, the more horrible their trauma must have been to mitigate, if not excuse, their own immoral actions. When we constantly try to rehabilitate the reputation of villains, we're moving responsibility for evil away from the individual who does the evil, and onto the institutional and environmental factors we think made the human monsters who or what they are.
The logic behind those psychological explanations for evil is weak. It's possible to suffer greatly and not become a (mass) murderer. That ...