Z is for Zealot
Zealots make the best villains. Their fanaticism means they’ll go to almost any extreme to accomplish their goals. These extreme actions make them more likely to come to the attention of the player characters, and more likely to call the players to action. While most Dungeons and Dragons campaign settings include a wide selection of gods and religions, religion is just one form zealotry can take.
A ranger or druid could decide to begin killing humans in a farming community because they’ve been cutting down trees in an ancient forest to build their homes. Leaders of the merchants’ guild kidnap the child of a politician who supports increasing their taxes. A necromancer is stealing every body from the local cemetary as part his plot to take over a kingdom, or even the world.
In addition to their willingness to do anything for their cause, their fanaticism leads them to overestimate their own abilities. This overconfidence can frequently lead to the villain’s downfall.