General:
* Antagonist
* Nemesis
* Scourge
* Tyrant
* Despot
* Fiend
* Monster
* Evil-doer
* Wrongdoer
* Criminal
* Miscreant
By Motivation:
* Power-hungry: Overlord, Dictator, Warlord, Emperor, Kingpin
* Greed: Thief, Robber, Pirate, Swindler, Plunderer
* Revenge: Vengeful spirit, Avenger (in a twisted sense), Retributionist
* Madness: Lunatic, Madman, Psycho, Sociopath, Psychopath
* Control: Manipulator, Puppet Master, Mastermind
* Ideology: Fanatic, Extremist, Zealot, Cult leader
By Appearance:
* Supernatural: Demon, Devil, Wraith, Ghoul, Spectre
* Unnatural: Mutant, Freak, Abomination, Horror
* Physical: Brute, Thug, Goon, Henchman
By Skill:
* Deceitful: Deceiver, Liar, Fraud, Conman
* Manipulative: Schemer, Mastermind, Trickster, Charlatan
* Brutal: Butcher, Assassin, Slayer, Killer, Murderer
Other:
* Intimidator: Threat, Menace, Terror
* Shadow: Dark figure, Phantom, Specter
* Mastermind: Architect, Orchestrator, Strategist
Remember, the best noun to describe your villain depends on their specific characteristics and motivations.