Names to Run Away From Really Fast/Xtreme Kool Letterz

A form of Names to Run Away From Really Fast: Xtreme Kool Letterz make an otherwise-ordinary name scarier. Most people with names that haven't been legally changed to have loads of "Kool Leturz" are best kept an eye on.

Anime and Manga

 * Xellos (Merry demon from Slayers)
 * Xanxus of Katekyo Hitman Reborn is the incredibly badass leader of the Varia, an elite assassination team primarily employed by the most powerful mafia family in the world. He has some amazing full-body scars, and he spends most of his time throwing blunt objects at his subordinates and breaking their faces with tables.
 * Subverted with Zagato of Magic Knight Rayearth . It seems at first that he is the main villain, but then the plot twist happens.
 * Izumi Curtis from Fullmetal Alchemist.

Card Games

 * Niv-Mizzet (of Magic the Gathering's Ravnica block; a dragon who was also a Mad Scientist, so you know he's dangerous, and in combination with a single card he can start an infinite damage loop)
 * The whole plane of Phyrexia.

Comic Books

 * Drax the Destroyer from Marvel Comics
 * All of the Gauls from the Asterix comics have an X at the end of their name.
 * Stryfe, clone of Cable.
 * Sleez, who had Superman and Big Barda star in a porno.
 * Grotesk
 * X-Force villain Crule

Film

 * Hugo Drax from the James Bond novel and movie Moonraker
 * Xeno (Xenomorphs, better known as the aliens in Alien) This one might be averted by simply being proper nomenclature. "Xeno" means stranger, and it has a very strange morphology.
 * Xander Drax, of The Phantom. Who? "X-A-N-D-E-R D-R-A-X, Xander Drax, begins and ends with the letter X."
 * The title character of XXX, though at least half of that is from being played by Vin Diesel.
 * Drexl (from True Romance)

Literature

 * Galbatorix from the Inheritance Cycle is a name you give you child if you want him to grow up to be an evil emperor. It's possible that Galbatorix chose the name, but why is beyond most people; it doesn't roll off the tongue and sounds generically evil. You'd think someone who wanted to take over the world would ease people into it by picking something which sounded nicer. And if Galbatorix is indeed his given name, then he is fully justified in wanting to kick some ass--which raises the question of why he was let into the Dragon Riders.
 * Pretty much everyone from Sanctaphrax, but Orbix Xaxis deserves a special mention.
 * Lampshaded in Unseen Academicals with Dr. Hix, who changed his last name's spelling from Hicks to Hix because a man with a skull ring would have to be mad, or shall we say, even madder, to pass up the chance to have an X in his name.

Live-Action TV

 * T'raltixx (From Farscape, with a Punctuation Shaker to boot)
 * Xena (the Warrior Princess). It doesn't hurt that "Xena" can be translated as the feminine form of the (Doric) Greek word for "stranger" (would be "Xeni" in Attic or Koine Greek), therefore placing her in several other categories of Names to Run Away From Really Fast.
 * The Young Ones gives us Vyvyan. You think the mere spelling of his name makes your head hurt? Just wait ‘til you meet him.

Newspaper Comics

 * There's an archvillain named Axel in Little Orphan Annie, of all places. Besides, it almost sounds like axe.

Tabletop Games

 * Drizzt Do'Urden; generally a nice guy, but you wouldn't like him when he's angry.
 * Jarlaxle, also of Forgotten Realms. He probably won't kill you. He'll just make your life a living hell.
 * Zaknafein, Drizzt's father, a four-hundred-year-old weapons master who's defeated demons in single combat and can take someone's tongue out with a whip while blindfolded. And depending on the person, enjoys doing it.
 * Warhammer 40,000: Ghazgkull Mag Uruk Thraka. An Ork over twenty feet tall. Run.

Video Games

 * This also applies to Axel Almer in Super Robot Wars Original Generation 2, but not so in Original Generation Gaiden, seeing that his non-OGs OG 2 personality is quite of a Jerkass, and a dangerous one at that.
 * Sort of applies to Axl of Mega Man X 7, but that's to the audience only anyway.
 * All of the Organization XIII members in Kingdom Hearts have names that are anagrams of their human names with an "X" added, like Lea-Axel, Braig-Xigbar, Dilan-Xaldin, etc. Xehanort follows the same pattern - among many other possibilities, his name without the X is an anagram of "no heart".
 * Syn, the Big Bad of Cardinal Syn
 * Mortal Kombat kharacters inklude Stryker, Kobra, and Havik.

Web Comics

 * Xykon, self named for the Xtreme Kool Letterz quotient. Gets (more) homicidal whenever someone spells his name with a Z by mistake - . Note the suspicious similarly of the name to the compound Zyklon B, used by the Nazis in death camps. Presuming this was intentional, it provides a good clue to Xykon's general nature.
 * X. Who's birth name is Elzandra, legal name Alexandra, and happens to be a Child of Prophecy. She also likes explosions. The only reason she wouldn't qualify is her extreme apathy.

Web Original

 * Dan Byker. Byker is not Biker.

Western Animation

 * Vilgax (the Big Bad of Ben 10)
 * Nebulans from the original Transformers series often have names like this. See, for example, Grax, Zarak, and Krunk.
 * Thrax, the villain of Osmosis Jones. Presumably intended to invoke "anthrax".
 * Xanatos
 * Pretty much everybody from the Fire Nation has a Z in their names. Zuko, Zhao, Azula, Ozai, Sozin, Azulon. Worth noting: most the Fire Nationals who don't have a Z in their names were never really antagonists, such as Iroh, Ursa, Piandao, Roku, and Jeong Jeong.
 * The Sequel Series, The Legend of Korra has Tenzin, Aang's and Katara's son and Korra's Airbending teacher.
 * Katz.
 * Kardiak, Kyd Wykkyd, and Wrex from Teen Titans.

Real Life

 * And don't forget Elron's favorite, Xenu.