If we look at the basic struggle of Good versus Evil, every Dudley Do-Right will inevitably face his Snidely Whiplash, usually tying Nell Fenwick to the railroad tracks before twirling his mustache. Now at the game table, that would hopefully be your players (a stretch of the imagination from Dudley I'm sure...) preparing to stop the Evil Lich/Vampire/Wizard/Tyrant from feeding the Distressed Damsel/King/Loved One/MacGuffin to the Volcano/Dragon/Vermicious Knid/Sloar. Granted not every plot will feature a villain, or otherwise distinguished antagonist, but those that do may remind us of an old saying: A Hero is only as good as his Villain.
So what goes into a villain? Every DMG will have some advice on this, but I still want to explore this idea. Perhaps maybe construct a thought process for this, or at least ask a bunch of questions that I don't intend to answer. (note: I'm going to keep using the word villain even if/when the term antagonist is a bit more appropriate. It'll be the more likely case when talking RPGs, so I'll just go with it.)
How Bad Is It?
The first thought when building a villain, for me at least, is how much of a villain are they? It's a simple question that may have a decidedly complex answer working on multiple dimensions. It's a question of scale. Assuming you've decided on the scale of your overall campaign, this one is tied more to the scale of your story arc. That may be the campaign as a whole, or a major story arc lasting several sessions, or even just a small single session throwaway villain. Not every Big Bad is THE Big Bad. Is it a major villain? Or just a flunky? Maybe it's some rogue element that's all on it's own. Agent Smith from The Matrix was a subordinate who went rogue to become a major villain by the end of it all, for example.
Let's not forget what the villain does. How bad are it's crimes? What kind of atrocities do they commit to be branded villain? Are we talking Aspercel, Destroyer of Worlds? Or that mean ol' Mr. Jenkins down the street who won't give back the kids frisbee? Do they want to watch the world burn? Or just ruin your day? How far will they go to achieve their goals? How much time are you going to devote to their machinations?
The important part may just be how these factors line up against one another. A single session, world destroying villain ends up as a surprise boss battle like in some video games, while a campaign villain who takes candy from babies for 43 sessions may seem downright silly. Does this work for your choice of tone?
What The Hell Am I Looking At Here, Wade?
Now that we know what sense of scale we're working with, exactly what is our villain going to be? Some undead monstrosity? A dragon? A miserly old man? An old undead dragon man? This part happens first for a lot of people in the mindset of 'this thing looks cool and I want it in my game'. It's a fair and common thought, and one I like to indulge. This is usually a no-brainer in most cases because of that., but it's also an opportunity to mess with expectations. Who would ever suspect the elderly baker's wife of being the serial killer? Who'd have thought that the ghost was actually an advanced AI using smoke and mirrors to frighten kids away from his server farm? And since when do draco-liches go around stealing everyone's left shoe? Whatever your inclination, make sure to have fun with it.
Ready And Willing, But... Able?
Competency. How competent is your villain? Bumbling minion or super genius? Genius minion ready to backstab a bumbling tyrant? Does your villain know your PCs every move? Or does he regularly make a fool of himself? Did he miss it by just that much? This part is often overlooked as we want our villains to be threatening at all times. Don't be afraid to throw in something less than god-like. Team Rocket has just as much place here as Professor Moriarty, but you want to pay attention to how this reflects on your PCs. Nobody wants to get punked by Corky, but omniscient villains often seem like cheating... even when they are. This also goes back to tone and duration. Recurring incompetent villains make for decent distractions, but may end up as an anticlimax if used carelessly. On the other hand, how much of a genius could the other guy be if we outsmarted him in a day or two?
Underhanded And Overall
Mixing things up a bit from villain to villain, arc to arc adds an uncertain element to things, spicing up even the most vanilla campaign. As always, season to taste.
Get down with your bad self and Stay Classy!