Here's an update taken directly from my notes....
Villains are the most diverse and often the most interesting characters in fiction for many reasons. My personal favorite stems from the idea that the villain can express things the author couldn’t, that the rules don’t apply to them, so they are free to act without limitation. I find that incredibly interesting and satisfying, particularly because it gives me permission to express ideas such as murder, lust and anger, where I would otherwise be unable to do so in person or in a more positive character.
Whether your villain is an intergalactic warlord, the most infamous serial killer, the little old lady who poisoned the vicar or the neighborhood bully, all villains are built on a solid foundation of principles. I outline three below, but many more exist.
1. Villains are built just like any other character, but with a different philosophy from your protagonists. Follow the previous steps for character construction, but be sure to pay particular attention to the philosophy the character expresses and demonstrates. The villain’s motives and ideas may not and need not be ultimately dissimilar from the hero’s plans, but there will be a wholly different approach, and a different interpretation of the outcome.
2. Villains have a more immediate view, but with a much wider scope. A good villain is someone who is very present-minded, very much aware of what’s going on within the plot, and very savvy to the immediate concerns. But from this direct focus comes a huge array of extrapolations about the future. This is best represented in action of fantasy villains where they’ll use a Macguffin (The Ring, The Ark, The Grail, etc) and take over the world. These extrapolations define the “stakes” for the character.
3. Villains will cross whatever line the hero won’t, and don’t normally consider it a great and profound feat to do so (or vice versa). A serial killer will butcher victims while the police won’t. The stuck-in-the-mud conservative parent won’t approve of the interracial relationship. Whatever the issue, the villain takes the opposite view. This is not necessarily true in all cases and with all thoughts, as the villain and the hero may be of similar minds on several topics, even they never get expressed. But for some issues, often the ones at the heart of the emotional arc of the story, the villain and hero sit opposed, and for good reason – this allows emotional investment by the reader and gives us someone to like and someone to despise.
Examine your favorite stories and see if the villains there follow these (and other rules), and employ them yourself.