What makes a villain? Is it the tactics they employ, the wages they pay, the acts they do or something else? From a role-playing perspective, the answer to that question is the abstract that gives them incentive to do evil. Since your players will most likely encounter them, why not give your villains some sort of calling? Five basic elements of that calling are greed, vengeance, zeal, sociopathy and misinformation.
Greed encompasses a great number of villains. Whether it's for might, power or money, many a villain is a villain simply because they want it all. It compels people to take more than their fair share and go to great lengths to keep others from having the object of their desire.
From an early age a villain can exhibit strong greediness, which means by the time your heroes meet them they're too far gone to be reasoned with. Perhaps not, and the party could help them see the error of their selfish ways. Regardless of your plans for them down the road, greed will fit as a driving goal for any campaign setting.
Vengeance is a slippery slope. On one side we've got avenging heroes who are making justice of old crimes. On the other side, villains overwhelmed by the drive for revenge for perceived wrongdoings. Note that word 'perceived' – an action doesn't have to actually happen to spark revenge.
The scary thing about this slope is that the line bisecting the righteous hero and terrifying villain is so narrow. Many villains of this type may have been heroes at one time but strayed too far from the path. Merely being vengeful, be it revenging or avenging, wards away the great pacifier known as forgiveness. Without that natural outlet it's a wonder more avenging heroes don't become villains.
Zeal carries with it a type of treachery. For some, absolutely devoted passion is a strength. With others it's little more than a trap. This type of passion can block us off from anything we don't know and people often fear and repel what they don't know. Enough zeal could drive someone to be xenophobic and destroy anything which is not like them.
Any passion can rob the senses. Interestingly, passion can also monopolize those very same. Something that was once a harmless pastime can, given the right circumstances, spawn an individual transformed. Examples of this include villains who spread their twisted ways onto others or the antagonist who is too wrapped up in their acts to understand the people being hurt by them.
Sociopaths can make impressive villains too. Possessed of a disconnect from compassion, some individuals wouldn't think twice of what empathic or sympathetic individuals would consider heinous. That disconnect could be enough to lead an otherwise peaceful individual to more and more cunning and devious acts. Whereas most of us have a conscience directing us and keeping us from being villains, some lack such an evolutionary check.
The opposite of the zealot, sociopathic villains exhibit more cunning than passion. Whereas the zealot would attack the king or lead an army against the town with fervor, sociopathic villains don't have that sort of wellspring to draw from. Instead, these villains view others as any non-sociopath would view an ant or other insignificant creature.
Misinformed villains, in my opinion, make for the best villains to role-play. These are people who go to evil because they are either misled or simply ignorant. I like using them because it introduces a level of subjective morality into my campaigns: Instead of “that thing is evil so we kill it” objectivity, the players must figure out if the villain is truly an evil creature or if they can role-play with them and gain a benevolent ally (after converting them from evil, of course). Misinformed villains make for the best role-playing experience because they can turn into reformed allies or even heroes.
An examples of a misinformed villain includes characters who are unwitting pawns in large evil schemes. Especially if that scheme seems benevolent to them, given their limited view of the whole. Most interestingly, these characters might see themselves as 'heroic' more often than other villains because they aren't grasping the full effect of their role.
Many of these can overlap. Vengeance and zeal would work very well together, for instance. If you've never tried the misinformed villain I highly recommend it – few encounters can get into as much character development as those. However, for those DMs entertaining ruthless hack-and-slashers, you might spare that type of antihero. No matter the villain you prefer, complexity makes for great role-play. Happy antagonizing!