The Villain: Twisted Hero or Evil Incarnate?

“No one is an unjust villain in his own mind. Even - perhaps even especially - those who are the worst of us. Some of the cruelest tyrants in history were motivated by noble ideals, or made choices that they would call 'hard but necessary steps' for the good of their nation. We're all the hero of our own story.”
― Jim Butcher, Turn Coat

One of my favorite quotes from urban fantasy author Jim Butcher expresses the concept of evil, not as an absolute, but as an expression of the villain's motives and psyche. Does evil ever exist for the sake of evil? When I think of my favorite books, the villain always has a "human" element, or a driving force that is real and just in his own mind. He doesn't usually see himself as the villain, but as a new and powerful wave of change.
Villains can be forces of nature. We don't consider typhoons, tsunamis, hurricanes, earthquakes, tornadoes, or other natural destructive elements inherently evil, but they do create havoc and chaos for those caught in their wake. Is the bad guy like one of these acts of Mother Nature? Is he an uncontrollable and unstoppable force because of his nature or does more appear under the surface?
As the creators of these evil phenoms, writers in books and film often desire to explain the villain, to look for the seeds of his corruption, or to have him represent an abstract concept. Like Jason Voorhees of the Friday the 13th series, perhaps the villain's transformation to mad killer is rooted in the bullying and abuse he suffered as a child. Or perhaps he is a representation of the neglect children suffer as Robert Englund says of the evil he portrays in the iconic character of the Nightmare on Elm Street series, Freddy Krueger. No matter what the purpose, the desire is to interpret the evil in some explainable way.

But, what about the evil for the sake of evil? Can evil ever be independent of the villain's past or the author's representation of theme? Even in the Paranormal Activity films, the demon-ghost haunting the family needed a motivation behind it. The terrorizing of the family was explained by a bargain struck between the ancestors years ago to claim the first male child. What if that reason didn't exist? What if the demon simply struck because he could? If rationalizations for evil don't come into play, does it give it more power or make it more terrifying?
Villains with motivations and past experiences that explain their actions make them round and intriguing characters. But, what if they don't have a motive? What if like a tsunami, they simply act and drown their victims in the force of the wave? Might that not be the most terrifying villain of all?

What is scarier to you: a villain who believes himself the hero of his journey and/or reasons for his motives, or an evil that is a force of nature with no true motivation besides destruction?