Evil is kind of complicated. There's evil actions, like murder, torture, terrorism, and the like. Then there is evil ideals. A lot of "dark-side" force traditions have ideals that are inherently either promoting chaos or evil. The Sith code (most of them, as the codes are often re-written) tends to believe in ambition and power above all else. Certain sorcerous traditions in the Galaxy actually espouse death as an object of worship, believing strongly in a desire to kill. Others' have motivations less about the self, and more about the state of the galaxy or other affairs. Some of the biggest crimes and atrocities on this forum have occurred because of IC personal vendettas. The fall of some major interplantary factions can be traced back to vengeance for a single murder.
One approach to villains is to have a character with a reasonable idea of the problem, and a genuinely horrifying solution. Good examples of this are Magneto (who responds to prejudice with violent terrorist attacks and no concern for collateral damage), a lot of other comic supervillains, and that sort of thing. In general, these fall under villains who have reasonable motivations, but are unreasonable in their means to achieve them, and that can be a compelling way to do a villain.
Another sort of villainy is evil as a force of nature. This works better in fantasy, but we happen to be in a fantasy/sci-fi setting where evil is a tangible force, if you'll pardon the wording. The dark side of the force is an existent thing that acts on people and places, that physically can contaminate and corrupt people, creatures, things, and even whole locations. While being evil as a consequence of proximity to evil can make for some good writing (especially if you want to explore the player's conflict with themselves over it), focusing on motivation becomes less meaningful, as the motivation is more of a compulsive behavior.
Monsters are also a type of villain. Some beings aren't touch by evil, they just are evil. Demons. Sithspawn. The risen dead. Some creatures have an innate evil by virtue of their very existence. I don't tend to feel innate evil is as easy to make compelling, and usually the most interesting stories about innate evil as a force come not from its evil for the sake of it, but from either the conflict it generates, or the consequences of its power. This kind of evil is usually more meaningful as a prop than as a character. A good example of this is Sauron in the Lord of the Rings. He's not compelling in his own right, but he's powerful enough that the consequences of his existence, and how others deal with him, allows him to be a driving force in a compelling story.
Villains can also be noteworthy by specific behaviors, that can produce a flavor. As Connor Harrison pointed out, appearance and quirks can be part of it, but so can theme and style. One of your examples is Lecter. Lecter isn't particularly powerful, nor is he all that unique in his actions. The reason he leaves such an impression upon the audience is because his actions and his behavior seem discordant at first. Cannibalism and murder seem ruthless and savage, and surely this genteel, soft-spoken, high-speaking academic wouldn't be such a depraved, inhumane monster? Hannibal Lecter isn't scary because he's a monstrous murderer with a taste for human flesh--Hannibal Lecter is scary because he talks like a guest lecturer at a prestigious med school right before he kills you and turns your organs into a seven-course meal.