NPCs you love to hate
Most D&D players love to hate NPCs. A typical characteristic of villains is that they are very intelligent but lack the humility to hide it. They will shove their knowledge and wits in everyone’s face.
Cockyness made easy
An easy way to make this come across is by using a fancy vocabulary. For cool words you can peruse the phrontistery, a website dedicated to obscure words and vocabulary resources. I would say more but I want to avoid logorrhea when possible! (see how annoying it is?)
Have fun being irritating when you impersonate the villain during your next game!