A Sinister Clue

A very old trope, in which left-handedness is a signal that that someone is the Villain or The Mole, or occasionally just susceptible to the Heel Face Revolving Door. The word "sinister" actually is derived from the Latin word for left-handedness, and also related to the Bar Sinister which is a symbol of bastardy, and thus untrustworthiness.

This is also related to Evil Twin; twins do often have opposite dominant hands, and an Evil Twin will (almost invariably) be the left-handed one.

Additionally, in older times, left-handedness was often a sign of evil or trickery because of the fact that people would shake hands with their right, grabbing each other by the wrists, in order to show that they were friendly and didn't have weapons. A lefty could shake with their right hand, grasping the other person's dominant hand, and attack with their weapon in their left hand while the other was defenseless. Also, during the Middle Ages, towers were built so defenders would have no problem swinging their swords down staircases while attackers would have loads of problems swinging up. However, this only applied if both were right handed; if the attacker were left-handed, the two would be evenly matched.

In many cultures, before the invention of toilet paper and forks, the right hand was used for eating, greeting etc., while the left hand was used for wiping oneself and other less savory things.

Another related trope is for a character to lose his right hand, symbolizing his loss of a moral compass. This can presage a Face Heel Turn by the Hero. It is also common for Mad Scientists to have a mechanical right arm. Alternatively, the left hand might become the repository of Sealed Evil in A Can. Compare Red Right Hand.

Note that this is not the same as The Killer Was Left Handed, because in that trope there is actually evidence that The Killer Was Left Handed, while in this case being left handed is simply a subtle form of Color Coded for Your Convenience.

The opposite, when left-handedness is viewed favorably would be The Southpaw. Also see I Am Not Left Handed.

Examples:

Anime and Manga

 * Figures prominently in Read or Die. Really prominently - will ruin the main plot twist if you read the spoiler.
 * Wrath of the first Fullmetal Alchemist anime may or may not be left handed, but he got the right one from someone else. The same could be said of Scar, though their alignments turn in opposite directions.
 * Gin from Detective Conan is left-handed. Also, there is a case Conan solves by realizing a man had been replaced by a duplicate because the fake was left handed and the original was not.
 * Higuchi in Death Note, though nobody mentions it. Light's ambidexterity probably counts as well.
 * One Piece: An interesting case with the Straw Hats, as none of them are evil, but a large number of them have something of importance on their left side. Luffy has his scar underneath his left eye. Nami's tatoo is on her left arm. Zoro's earrings are on his left ear, and although ambidextrous appears to prefer his left hand. Usopp always wears his wrist band on his left hand. Sanji's left eye is always covered. Chopper's antler plate is located on his left antler. Interestingly enough, the two Strawhats that finally break this pattern, are both introduced to the story as villains/antagonists to the Strawhats.   then completely subverts it, as his one asymmetrical trait is on the right side of his face.
 * Inverted by Nanoha, the left-handed heroine of the Lyrical Nanoha series, which is full of right-handed villains. Unless, of course, you take her Fanon reputation into consideration...
 * In Naruto, Sasuke realizes that the Naruto in front of he and Sakura is an impostor because his knife pouch is on his left leg, implying left-handedness, while the real Naruto is right-handed.

Comic Books

 * Green Lantern villain Sinestro wears his power ring on his left hand.
 * As does Alan Scott (the Golden Age GL), inverting this trope.

Film

 * Keyser Soze in The Usual Suspects is left handed. A clue to his identity occurs early in the movie, when.

Literature

 * Arya from A Song of Ice and Fire. When her dominant hand is revealed to her fencing master, he reacts favorably because fighting left-handed will reverse her stance and movements, which will help confuse her opponents. However, her left-handedness might also be A Sinister Clue about her future,
 * In the novel The Sterkarm Handshake, the whole Sterkarm clan are lefties and are known for being backstabbers.
 * In The Silmarillion: both Maedhros and Beren lose their right hands. This trope is probably averted with Maedhros (though he's something of an Anti-Villain, he remains fairly decent for a long time after losing his hand) and definitely averted with Beren, a wholly positive character.
 * In The Bible, Ehud, one of the judges, actually managed to assassinate a king by hiding a miniature weapon on his right hip. Thus the weapon wasn't found, and he could still use it, being left-handed.

Live Action TV

 * Played for laughs in some comedy (?) show called Inside Scoop: Ban Left Marriage!
 * In V, whenever a human is converted to the villains' cause, it causes a brain-dominance switch. Since There Are No Lefties in TV Land, this leads to left-handed human collaborators.
 * In Star Trek the Next Generation, Data figures out he and Picard are trapped in a holodeck Enterprise when (the simulation of) Geordi catches a tool Data throws to him with his left hand, exposing a programming glitch Data was already aware of.
 * That one actually bothered me, even in the Sherlock Holmes story alluded to in this episode. Think of baseball players. They are (like most of the population) right handed. Their mitts are, therefor, on their left hands so as to allow them to keep their dominant hands free of obstruction.
 * You're not alone: a reader of the Nitpicker Guide apparently did a half-blind experiment where he threw a ball at people and found regardless of which hand they were dominant in, they caught the ball with the most convenient hand (it depended on which angle it was being caught from).

Tabletop Games

 * Paranoia: "Left-handedness is one of the ten early warning signs of Commie mutant treason!"
 * Inverted (thematically at least) in the Lone Wolf gamebooks: magic traditions inspired by the Forces of Light are referred to as "left-handed", while the Black Magic of the servants of God of Darkness Naar are referred to as "right-handed".

Video Games

 * Dyne, a side-villain in Final Fantasy VII, had his crippled LEFT arm replaced with a machine gun, and is distinctly portrayed as a serial killing lunatic. The reverse is true of Barret - even though he leads a terrorist organisation that blows up a power plant and kills hundreds of people, his RIGHT hand was the one replaced with a similar weapon. He turns out to be a nice, but misguided person who is a doting father and dedicated friend.
 * Sephiroth and his remnants are also lefties.
 * Kain Highwind, a party member from Final Fantasy IV, is left handed, which is shown by having weapon and shield slots in his inventory screen reversed. No points for guessing what he does, especially with a name like that. Though he does get better... somewhat.
 * In God Hand, the titular artifact exists in a pair: left God Hand and right God Hand. The hero, Gene, got the right one to replace his severed arm. As for the left one, it's used by the Dragon Azel (who is arguably the most evil in the group of villains).
 * Barring the Wii version of The Legend of Zelda Twilight Princess (which was mirrored left-to-right from the Gamecube version), every game in the main franchise (until Skyward Sword had a left-handed Link.
 * In Super Smash Bros, Master Hand is a giant, floating, apparently benign right hand who created the world that the characters find themselves in, and lives to be the ultimate challenge for heroes. His Evil Twin is the Crazy Hand, the left hand of the pair, who laughs maniacally upon his entrance and whose attacks and general movements are both bizarre and incredibly erratic. Whereas Master Hand is said to be a benign force and to represent the creative nature of a child, Crazy Hand is a destroyer, and supposedly represents that same child's more sadistic qualities.
 * An odd example occurs in the online game Improbable Island, where one of the monsters you can encounter is called "Sinister Dexter", an Affectionate Parody of, well, Dexter. His moves are all based on being left-handed. It's that kind of game.
 * "Dexter" is also derived from the latin word for the right, or right hand, making the character name an obvious contradiction.
 * The official art for Castlevania: Symphony of the Night depicts Richter Belmont as a lefty; he is right-handed in the prior depictions of him. Hint hint much.
 * The two models of the portable Snag Machine are depicted as being worn on the left arm. Neither of their wielders are particularly nasty (though one moreso than the other), but the folks behind inventing the device are the epitome of evil in the Pokeverse.
 * As a comparison, in XD, Michael always makes a regular catch (at the Poke Spots, given that's where wild Pokemon appear from time to time) or calls out a Pokemon with his right hand. He only uses his left when he needs to make a Snag.
 * Not quite sure if it counts, but Bowser bats and pitches left-handed in Mario Super Sluggers.
 * Inverted in Tales of the Abyss, where The Hero is left-handed,

Webcomics

 * Casey and Andy: Satan is left-handed.
 * Narbonic: Caliban is left-handed.

Western Animation

 * In one "Treehouse of Horror" episode of The Simpsons, left-handed Bart turns out to be the evil twin of Hugo Simpson. Bart's left-handedness is never brought up in the episode though.
 * There's no indication as to whether either Bart or Hugo is left- or right-handed; however, Bart was the left-sided (or Sinister) twin.
 * Francine from American Dad was raised to think lefties were evil.
 * In one episode of The Batman, Mirror Master creates evil duplicates of Batman, Robin, and Flash. Batman quickly notes that the duplicates are left-handed.

Examples of the related tropes:

Anime and Manga

 * The left hand of the titular character of Hell Teacher Nube is actually a sealed Oni, Baki, which replaces the left hand he lost fighting said Oni. Most of the time, it looks like his regular hand wearing a black leather glove; when his students are threatened by spectral horrors, he will reveal it as a monstrously large, dark red hand with visible tendons and black claws. However, he constantly seeks a way to seal it permanently, as only the influence of his childhood teacher prevents the Oni from taking over his entire body.
 * The lead characters of Parasyte are a young man named Shinichi and the alien parasite that has possessed/replaced his right hand. Their relationship is one of the main foci of the series, especially re: Shinichi losing part of his humanity. As early as the first volume, the Love Interest intuits there's something weird about his right hand and switches around him to hold his left instead.
 * The original English translation was flipped and 'Migi' was named 'Lefty' instead.
 * Edward Elric of Fullmetal Alchemist lost his right arm and his brother lost his entire body performing forbidden alchemy, which put him on a self-imposed Redemption Quest to restore his brother's real body. At the end of the story,

Comic Books

 * Lex Luthor also had a mechanical right hand for a while; he lost the original to Kryptonite poisoning. Naturally, he took Rotwang's role in the Elseworld Superman's Metropolis.

Film

 * Reverend Harry Powell from The Night of the Hunter 1955. The fingers on his right hand are tattooed with the word L O V E, while the left hand is tattooed H A T E. His Catch Phrase? "Do you want to hear the story of left-hand and right-hand?"
 * In Star Wars: Attack of the Clones Anakin loses his right arm- symbolism of a move towards the Dark Side? Luke then loses his right hand in the same way- right before The Reveal that Luke, I Am Your Father, and in Return of the Jedi Luke breaks free from his Freak Out after he cuts off his father's right hand and realizes that both of them have mechanical right hands.
 * Rotwang Metropolis 1926, here we have the subtrope with the Mad Scientist whose right hand is mechanical- also seen in Girl Genius with Professor Mongfish.

Literature

 * In the books, Hannibal Lecter had six fingers on his left hand.
 * Inverted in A Song of Ice and Fire. When  loses his right hand, his moral compass actually starts working better. This is mostly due to the fact that without his sword-hand, he's forced to think things through, when before he could just hack his way through any obstacle.

Video Games

 * In The World Ends With You Sho Minamimoto has a black left hand.
 * The main villain of Final Fantasy IV, Golbez, is left-handed. However, you only learn this when . He wields weapons in his left and shields in his right. Similarly, is left-handed, and ends up brainwashed to fight for the villain.
 * Notably inverted with a lot of the characters Shigeru Miyamoto creates. Mario and Link are both left-handed, and Miyamoto himself is ambidextrous, preferring his left hand.