Strangely, my villainous characters are usually the ones mistakenly perceived as heroes and acting for the greater good. In some books, you have the people who you think are good but turn out to be double-agents and such, accompanied by those who you thought were bad but were the real heroes all along. In my stories, the villains are never really the villains: they are just hugely misunderstood. Heh, I understand that's an overused excuse, but in all honesty, they are. They know what they're doing, they know they are destroying cities and killing people, but they really are doing it for the greater good. In some cases, the populace believes them, but when they don't, they are automatically classified as the villains, and the special forces are sent to stop them "at all costs".
I prefer stories that have villainous heroes than heroic villains.
Those we identify with. Those who challenge thoughts (Smith from Matrix), Those who has a point (Adrian, Watchmen). Those whom we cannot understand (Alien, Alien - Joker, Batman). Those who aren't necessarily evil, as evil, in Kantian terms, implies irrational, and intelligence is an alluring trait in a villain. It can even be as little as merely opposing the protagonist in one form or another. In certain cases like Sauron (LOTR) or Vader (Star Wars) it can be the mere aesthetic and bad intention, but this is extremely hard to pull of and perhaps needs an epic to go along with it. Usually, ambiguity, nuance and complexity hooks me on much more than cruelty for cruelties sake.
Damaged past doesn't absolve villains of anything, it's no excuse. It just makes them easier to understand and provides a reason for them to do what they do. To make one convincing, they generally start out with good intentions (but didn't see the sign that said 'this way to Hell'), and then they just hit the extreme and it all goes downhill.
Villains are the best characters, because we all see a little more of ourselves in someone who does things wrong. We're more aware of our faults than our more heroic qualities. When I write, I like to blur the border of good/bad. The heroes should have a dark side if they're to be believable.
Evil people are often heroes in their own mind. They are doing horrible things, but to them, in their warped world view, it's perfectly acceptable for the greater good, or for their own well being, or maybe they simply lack a moral compass.
Evil makes me think of people who do bad things and enjoying doing them. Like they know what they're doing is harming others yet they do it anyways. They could also be misguided cos my evil character is sorta misguided. He was promised something if he performed cruel acts... of course, this character has a bit of a mental problem as well...