I don't believe in evil. It's a word I use, certainly, because words are shortcuts and we all take the short way round from time to time, but that's all it is, a word to describe something we cannot or will not understand or articulate in any other way. If there were such a thing then I would have a much harder time condemning acts that I find selfish, cruel, or otherwise indefensible, because to say that evil exists is to provide an excuse for what I find to be obvious: that humans are animals, and as such often act in ways that other animals can never fully understand.
The first thing I thought when Powell's offered me a soapbox was, Don't come off like a pretentious jackass, be sincere, talk about something you love, like hiking or Vincent Price or Jack Vance or Umberto Eco, and next thing I know I'm using a Milton quote for the title and waxing on about philosophical uncertainties. I'm going somewhere less bombastic, really! Evil as human weakness versus evil with a capital E as a means of segueing into why I've always loved villains, not as a clumsy rant about ethics.
While most "you're either this or you're that" demarcations have the ring of bunkum to my tin ear, I do think there is one sweeping generalization that holds true: you are either interested and willing to hear the villain's side of things or you're not. I am, and so are a lot of people; there's a reason why Dante's Inferno is far more popular than the latter two parts of the The Divine Comedy. Whether my personal interest in villainy is the result of my feelings about the nature of evil or simply because I watched a lot of horror movies as a kid is irrelevant to the point I'm dancing toward: I love villains.
Vincent Price and Christopher Lee were always more interesting to me than Peter Cushing, grand as the Cush most assuredly was, and growing up I couldn't get enough of monsters both human and, er, monstrous. The makers of the Price films definitely understood this, and often provided a thin little scrap of plot to explain Price's wicked ways and give the audience an out to root for him: the so-called "abominable" Dr. Phibes would never have imaginatively dispatched all those doctors if they hadn't killed his wife. Sure, the death of Mrs. Phibes might have been an accident, and granted, covering a woman in honey so that locusts will devour her isn't really a constructive way of dealing with grief, but as a kid my sympathies were always with Uncle Vincent.
Until Witchfinder General, that is, marketed in the U.S. as The Conqueror Worm to cash in on the Price/Poe craze. The film, about the infamous, Commonwealth-era witch hunter Matthew Hopkins, sucker-punched me. In one of his least hammy roles Price is actually chilling, and for the first time I recoiled from him as one would a stinging insect. Yet I was just as fascinated as when I was cheering him on in Theatre of Blood, and to date Witchfinder General remains one of my favorite Price films. As a child I couldn't quite articulate the difference between a villain I could root for and one I couldn't, but I think now it comes down to my predilection to side with the underdog. A weak villain, one without agency or other recourse, is one I can and did get behind, but a villain like Matthew Hopkins, someone who had power and exploited it to satisfy his sadistic desires, is far less sympathetic.
Not that a villain need be sympathetic; indeed, the difference between hero and villain is often one of perspective, and even when the storyteller establishes a dichotomy between the characters based on the audience's conceptions of good and evil, we are still drawn to the villain, to the mysterious motivation that we cannot understand or sympathize with. Not everyone feels this way, as I said, and many a storyteller is content for their villain to be a bland caricature, but we recognize our own, we villain-sympathizers, and we often seek out something to sympathize with when we like a villain, even if involves a bit of stretching. Sometimes the storyteller makes it easy, sometimes the villain is even the star, but more often than not we're left following around some Goody Two-shoes until the character we're really interested in shows up to menace the hero.
An innocent young woman is menaced by a monstrous villain, and after much physical peril she is saved by virtue of her virtue. This is the fashion in which mainstream audiences seem to prefer their villains displayed, as an object to contrast against the righteous protagonist while providing some cheap scares. Fans of contemporary horror cinema and fiction could doubtless catalogue an enormous list of titles that more or less follow that plot, and even John Milton employed that formula for his masque Comus.
In Paradise Lost, on the other hand, Milton took the very different approach of featuring the villain as protagonist, and, arguably, a sympathetic character to boot. This shift from villain as Evil foil to villain as complex, thinking individual at odds with convention caught Milton quite a bit of flack at the time; the difference between glorifying a villain and exploring his personality to better understand him is a distinction that can be difficult to explain to critics. Not that Milton was first or the last to focus on the villain and portray him in human, as opposed to strictly monstrous, terms; Marlowe's Tamburlaine and Doctor Faustus, Shakespeare's Macbeth and Richard III, hell, in almost every literary movement (particularly the Gothic)