I have trouble writing villains

I have no problem with description, with plotting, with nifty narrative, but when it comes to writing a truly convincing, internally coherent villain, I suck. Maybe I ought to write a story about the villain, one in which he becomes more villainous and not less, (or she, let's be real) and really work on motivations and character for him/her. As an exercise. Just for fun (or for profit).

Too many stories I really want to write are lacking in suitable villains. It's also why I was a lousy GM. Sigh. Done noodling for the evening. Really want to get some sleep, but not sure I can shut off my g-dmd brain.