This prompt! It was made for me! I'd like to point out though that in this prompt especially I do make a distinction between 'villain' and 'antagonist'.
1. What makes a good villain, in your opinion?
For me, the biggest thing is: three dimensions. Three dimensions three dimensions three dimensions. A villain who only has one layer or motivation isn't a villain to me, it's a piece of cardboard. I've heard people say before that a good villain is somebody who doesn't know they're a villain, but I'm inclined to disagree; some of the greatest villains, both as antagonists and protagonists, have been (arguably) aware of their own evil. (Alex in A Clockwork Orange). And others, while aware of it, may seek to justify it all the way through. (Lolita's Humbert Humbert).
The biggest thing you can do is have a character who ceases to be a character and becomes instead a person.
2. Do you tend to write more for villains or heroes?
Villains. I can't remember the last time I wrote a hero--and even then, he was a Byronic hero.
3. Who wins more often in your stories, the good guy or the bad guy?
Well, I seldom write good guys even as antagonists. Generally I despise the term bad guy as a synonym for villain, because 'bad guy' implies the person who we're to root against--and beyond that, there's an issue of embracing black and white morality and the boringly two dimensional characters it makes.
Take Painted Blind, for example: the only possible "good guy" (who, if the characters' interpretations are to be believed, is not quite so good) in the book dies in the beginning of Chapter 4. From there the readers are left with Richard and Delilah for main characters, and supporting characters who range from despicable, to all too human. But I make certain that even the most vile characters (Richard, Susan) have some positive qualities, something sympathetic in them.
And the ending of Painted Blind is just as gray. Depending on one's point of view, either everybody loses, everybody wins, or everybody does both.
4. Have you ever written a redeemable / reformed villain? A good guy turned bad?
No. I suppose that to some extent some of my villains are "redeemable" in the psychological, if-they-took-their-meds-things-would-be-fine sense, but generally I feel it is up to the character to make themselves redeemable. Ideally, one finds oneself both repulsed by Richard, and yet intrigued or even admiring of some of his more creative, hopelessly romantic qualities. Likewise, I want for the readers to sympathize with and grow fond of Delilah, but at the same time horrified by her.
What should not happen, regardless of antagonist or protagonist, is that the villain's cruelty should not be applauded, or artificially softened and justified by the writer. If the reader does find themselves rooting for the villain protagonist, especially if said protagonist is oh, I don't know, a serial killer, then they should feel conflicted about it and be reminded that the crimes are never justified no matter the victim. Dexter can suck my nonexistant co-
5. Are there any themes among your bad guys villains- do you tend to write zombie stories, fantasy villains, etc?
Not really. I guess they're all just...people? I don't know. My main characters in the past three years have consisted of serial killers, voyeurs, drug addicts, necrophiliacs, Byronic demons, and vampires--and that's just in my solo fiction. In RP there've been many more.
6. Are some of your antagonists non-villains, just at cross purposes from the hero?
Uhhhhhhhh well...yes and no? Many vignettes I've written don't have any kind of antagonists, and Painted Blind itself is a very screwed up case. For example, both Richard and Delilah are arguably antagonists as well as protagonists--mostly for one another, although sometimes, usually in Delilah's case, for herself.
The main antagonist (in that she is shared by both Richard and Delilah, and is responsible for the greatest number of issues throughout the plot), however, is Susan, who is, like the protagonists, a villain. Then we have Michael, the aforementioned "good guy" who dies in Chapter 4, who, while perhaps emotionally manipulative and controlling, was not a serial killer; Samantha, who is something of a harmless, neutral force and becomes Delilah's main antagonist through no real fault of her own (although one may argue that her actions four years prior to the novel, trying to trap Richard with a pregnancy, was pretty shitty and underhanded though certainly not villainous); and Julius, Richard's main antagonist, his absent father who gives Richard all the prescription pills he wants in hopes that he'll overdose--but keep in mind that Richard is using his very existince among other things to blackmail Julius, and Julius' deep-seated fear that Richard will harm him or his family, and his actions become less pointlessly reprehensible.
So, again. Not every antagonist is a villain, not every protagonist is a hero--and the line between the two is (or should be) blurry as hell, anyways.