To write fully developed wicked characters, it helps to remember that villains are people too!
I’m not sure if there is an “art” to writing about villains, but I do find that to write convincing and three-dimensional villains, one must be sympathetic to their plight. I’m always drawn to stories where good and evil evil isn’t depicted in stark black and white but in infinite shades of grey. We need to understand what drives a person to do evil things in order to understand our own less than noble intentions.
When I was writing the "Isle of the Lost", I decided to have the protagonist, Mal, experience the same snubbing that befell her mother Maleficent. As we all know Maleficent was not invited to Princess Aurora’s christening, which set off the events in "Sleeping Beauty". In my story, Mal is six years old and not invited to Evie’s birthday party. This also sets off the events in my story, and it also gives Mal a little perspective into her own mother’s psyche.
Being rejected and excluded creates a pain that is as real as physical pain, and I think when you write about villains you have to remember that their desires and motivations are just as important to them as the heroes and heroines’ desires and motivations. The art of writing them is to humanize them, to me, they’re not these stick-figure dark lords glowing with a red eye on a horizon, they’re wounded and selfish people who are just trying to do their best to get rid of this pain they are feeling—by inflicting it on others.
In my vampire series Blue Bloods, the real villain of my story is a flawed hero, a failure. I love tragic stories, and I love stories about villains, I see our humanity reflected in their tales so much more clearly than the classic hero stories. Look at the current pop culture slate—Mad Men, Breaking Bad, House of Cards, led by anti-hero characters who are much more complex and interesting than a do-gooder hero.
Root for the villain, if anything, they’re the hero of their own story.