A form of Names to Run Away From Really Fast: Since "mors" is Latin for "death", and many Latin-derived modern languages use something with a close resemblance (French "mort", Spanish "muerte", Portuguese or Italian "morte", etc, or even worse German "Mord" for "murder"), any name with mor- or mort- can be used to indicate death or evil.
Morgenstern (Morning Star) is a particular variation that applies specifically to Satan; see also Louis Cypher.
Mörđr Valgarđsson, one of the few people in The Icelandic Sagas who come across as unambiguously "evil".
Mordred, Morgause and Morgan le Fay from the legend of King Arthur.
In J. R. R. Tolkien's invented Elvish languages, "mor" means dark or black, hence it often appears in "evil" names: Morgoth ("Dark Enemy", Supreme Big Bad), Mordor ("Black/Dark Land" of Dark Lord Sauron), Morannon (the "Black Gates" into Mordor), Minas Morgul ("Tower of Dark Sorcery"), Moria ("dark pit"). But mor also appears perfectly innocent at other times, e.g. in the girl's name Morwen ("dark/black maid", referencing hair color).
Sherlock Holmes' archnemesis, Professor Moriarty (and his right-hand man, Colonel Sebastian Moran). Word Of God has it that Moriarty was named after a real, extremely violent, criminal whose name Doyle spotted in a newspaper.
Harry Potter's foe Voldemort, meaning "Flight of death" or "Theft of death" in French. And if you say his name in Deathly Hallows, you really do have to Apparate. Really fast.
Bonus points to him for making it up himself with an anagram of his real name. —>Tom Marvolo Riddle = I Am Lord Voldemort
There's also the spell which summons his Death Eaters' symbol, Morsmordre.
As well as his relative, Morfin Gaunt, who has a tendency to nail snakes to doors.
Mort of Discworld is an exception, as despite being Death's Apprentice, he's a skinny little nervous guy...until he gets into his role...
Morkai the Red, a minor character but powerful wizard in the early Drizzt novels, who is vicious and dogged in paying back those who killed him.
Moridin from The Wheel of Time. Dangerously insane, third most powerful evil person in the world (after the Dark One and his avatar), name means "death" in the Fictionary of the books, and just to top it all off, his previous name: Ishamael, a.k.a. Ba'alzamon. Oh, like it wasn't obvious.
Mordeth from the same series. Bonus points for almost having "Death" in his name as well.
Mordion from Hexwood, who has the face of a skull and is tasked with killing political enemies.
Morgra from David Clement-Davies' novel The Sight.
The Neverending Story has the villain G'mork, the servant of the Nothing, and Morla The Ancient One, who, while not being a villain, represents the nihilism and atrophication which allows the Nothing to take hold.
Mabinogi has Morgant, also known as Dark Lord, who is both The Dragon and has yet to be defeated in the mainstream storyline; both times you actually fight him the dialogue suggests he let you win, although Tarlach does manage to knock him out temporarily in a cutscene.
Mordac, the Preventer of Information Services in Dilbert
Latinized version from Warhammer 40000: Mortarion, Primarch of the Death Guard. Also Mork.
The God of Death in Warhammer is named Morr. However, he's actually of the nicer part of death gods.
Subverted with Mortimer Brewster from Arsenic And Old Lace. He reviews murder plays for a living, but, unlike several other members of his family, he hasn't committed actual murders.
The first enemy the Toa Metru of Bionicle went up against was the Morbuzakh, a Matoran-kidnapping plant.
General Donald "Devil Rebirth" Morden from the Metal Slug games.
Gym Leader Morty subverts this. His name alludes to the fact that he uses ghost types - but he's a really nice and easygoing guy.
Albeit a bit cocky.note "It's said that a rainbow-hued Pokémon will come down to appear before a truly powerful Trainer. I believed that tale, so I have secretly trained here all my life. As a result, I can now see what others cannot. I see a shadow of the person who will make the Pokémon appear. I believe that person is me!"
Morcubus, Prince of Monsters, from the MySims series.
Mortimer McMire from the Commander Keen series, who has tried to destroy the Earth (Invasion of the Vorticons), the galaxy (Goodbye Galaxy), and the universe (if the Universe is Toast trilogy had been made, that is...)
Mort the undertaker in The Curse Of Monkey Island — though he's a Harmless Villain (if that) with exploitable superstition. He eventually helps you once Murray threatens to curse him with the inability to "find socks that match."
The initial villain in Tales Of Vesperia is called Mordio. However, this is subverted when it turns out that "Mordio" is an imposter and the real Mordio is one of your party members.
The .hack franchise has evil AIs Morganna and Morti.
Mors Westford from A Game Of Thrones Genesis is a Night's Watchman feared by friend and foe alike. His nickname? The Butcher.
The variation mar- appears in the English "nightmare" and the French equivalent "cauchemar". And in Esperanto "kosxmaro". It's thought to come from proto-Indo-European, which means this trope is some four thousand years old.