Malebolgia from Spawn
Chun the Inescapable ( from The Dying Earth; he wears a cloak of human eyes )
Zapranoth, a demon in Empire of the East
Vicious of Cowboy Bebop ( “Who ARE you ?” “Vicious.” )
THAT WHICH ABIDES, from Andre Norton’s Janus novels; always spoken of in all caps ( THAT, IT and so on )
The [COLOR=“Blue”]house[/COLOR].
Which one?
Oh come on, sure he’s a dick, but he usually cures the patient. 
No love for Snidely Whiplash?
The Nothing
KHAN!
Josef Mengele
The Midnight Bomber what Bombs at Midnight
“Our name is Legion, for we are many.”
Galactus
Ultron
Sinistar.
Shropshire Slasher.
On the subject of robots…
Bonecrusher
Devastator
I always liked Clarence Boddicker’s name. He’s my favorite movie bad guy ever. Maybe there’s nothing really sinister about the name itself, but it sounds really creepy because it’s associated with him. I dunno. But it’s my favorite villain name. (From Robocop, in case you don’t know.)
Fronm The Great Race
Professor Fate
Baron von Schtupp
Exitar the Executioner
Brother Blood
The Serpent in the Sun
Baldanders.
Since I just watched the Iron Man riff track last night, I’ll have to add “Obadiah Stane” to the list.
(did they consider calling him “menacing t. badperson?”)
Boris Badenov
And BurnMeUp beat me to Snidely! Way to go, I loved that character! 
Question: Is a person who performs dastardly deeds known as a DASTARD?
Q
Maleficent.