The Virtues of Villains
The writer C.S. Lewis once noted that to be a successful villain, one needed a strong virtue. This is a literary truism that bad storytellers tend to forget. We know our villains have to be people we are willing to hate, yet we often forget that some virtue is needed to become a menace in the first place.
Case in point: Darth Vader. A guy we love to hate. But he has virtues: he has self-control (he doesn't kill his lackeys wantonly) and leadership skills. One of the reasons why people weren't all that enthusiastic about The Phantom Menace is because they couldn't see in young Anakin Skywalker the vices that made him Darth Vader. But the virtues of Anakin--his passion, his reluctance to leave his mother--do start to morph into Vader's vices in Episode II.
Another case in point: Retsudo Yagyu from Lone Wolf and Cub. A major player who murdered Lone Wolf's family and framed him for treason, he nonetheless displays the virtues of a samurai in the later part of the manga series. He battles Lone Wolf with a sense of personal honor, looks after Lone Wolf's son during a truce, and treats his ninja subordinates with greater respect than we might expect from a typical Western villain. I'd argue that these displays of virtue make Retsudo a more well-rounded villain -- and thus, a more memorable character.
Think about the villains you remember from your favorite stories -- Saruman and Sauron from Lord of the Rings, Doctor Doom from Fantastic Four. the list is endless. Odds are, if you analyse them, you'll find some form of virtue that makes up a key component in their personality.
Is this important? Yes, because a virtuous villain is one capable of redemption. Darth Vader dies a hero because he rediscovers his virtues of love and self-sacrifice. It is that possibility that draws us, the listener of stories, to bad guys who can become good.