Writing Good Evil-Doers

Every story needs its villains, its antagonists. In terms of black and white...in every story, every struggle needs its evil to your protagonist's good. Now the actual terms of who is good and who is evil is mostly a matter of perspective, but a story's antagonist needs to be written well for the story to be convincing.

The antagonist might not always be a person, they might be a setting or an abstract event...but what any antagonist must be is an obstruction that causes conflict. When writing a villain for much of anything, I keep a few simple things in mind while creating them.

Who are they...that makes them so evil?
What about them makes them an obstruction to the protagonist? What are they doing during the course of the story that causes conflict? In essence, what does your audience see them as?

Why do they do it?
Why are they doing...whatever they are doing? What made them act in the way they now do? This might be a single "hinge" moment that directs their life or a lifetime long series of evils/atrocities committed against them. Keep in mind that quite a few (most) antagonists don't view themselves as evil.

What are they going to do?
What are their plans goals and aspirations? What kinds of things are involved? What's the scope of their ambitions? In essence, what is the long term plot of their life that draws them to interact and become a problem for the protagonist/s

How will they do it?
How are they going to enact their nefarious, and not so nefarious, plots? Are they a financial mogul? a sorcerer? A war-captain? a mindless murderer...etc. How do they pursue their goals?

Their villainy thus far!
What have they done so far? How close, or far, are they from achieving their goals? This will basically indicate the point in their life in which they cross the other characters in your story

The Nuts and Bolts
At the very end, add in some flavor to spice the antagonist up. Make them real and interesting.