A gadfly screws with other people because it's amusing. Others do it because they need their pawn to do something. Then there is the infamous Troll. What separates this character from a gadfly is that what they do is not supposed to be funny. It's just cruel. What separates a Manipulative Bastard from a Troll is that with the latter, there is no real point to their manipulations beyond hurting the victim. All they want to do is see someone suffer or make a fool of themselves.
This has recently been the subject of a notableCritical Research Failure taken Up to Eleven by British Newspapers as of mid-2012; the term "Internet trolling" they refer to is in factcyber-abusers or internet bullies, which is far worse than trolling. In any case, a troll is someone who would probably say "I like New York Yankees. X [team on fan site] suck!" in a baseball forum, whereas a cyber-abuser is not a troll and does it for more harmful reasons, which toy with emotions sometimes.
Ironically, trolling (the original kind, not the modern definition) is now on the way to becoming a Discredited Trope — although it still seems to be tolerated in football forums.
In any case, the proper term for this behaviour is cyber-bullying.
The term trolling comes from fishing (i.e. trawling), of all things. The idea is that you set out some bait and watch as your victim grabs it and writhes for your amusement. Malcontents on the internet protected by anonymity have been doing this for years, hence the widespread usage of the term. When encountered in a game, a troll is often called a Griefer. While the word and concept are based on these beginnings, the term has spread from there such that it has become a widely used term to discuss pointlessly cruel characters.
If they just want to see their victims flail, then they're a true Troll.
See also For the Evulz. Compare and contrast The Gadfly, who is relatively harmless, but can be easily mistaken for a troll on the internet. Contrast with Manipulative Bastard, as such a character has an ultimate goal more than simply making someone suffer for its own sake. A female troll is known as a trollette or a troll bitch. Supernatural versions of this trope can also be a Jackass Genie.
For an article about mythological trolls, see All Trolls Are Different.
The film of the same name is over here.
Sayonara, Zetsubou-Sensei has Meru Otonashi, who (true to her name) is extremely shy around others, and only communicates through text messages... pretty much all of which consist entirely of outrageous slander of the recipient. And don't even think about taking her phone away, because she has extras. And don't bother waiting for the battery to die. She has extras. Many extras.
Durarara!!'s Izaya Orihara not only trolls the internet (his favorite tactic appears to be posing as a girl in chatrooms) but also real life on a near-constant basis.
Hisa from Saki. Her initial reason for having Nodoka play with her stuffed penguin, Etopen, was to collect "joke material," on her, but she soon found that Nodoka having it at her side like she did when she played online made her play better. She also allows Momo to get the impression that something happened between her and Momo's close friend Yumi, causing her to freak out.
In Jay and Silent Bob Strike Back, the plot is kicked off by the duo's reaction to trolls on a movie review site. The movie also ends with them flying to each troll's house and beating the ever loving hell out of them.
Joker: Make me. It doesn't matter. I win. I made you lose control. And they'll kill you for it.
In Ender's Game, Peter and Valentine troll message boards in order to learn from the angry responses. Step 1: Troll message boards. Step 2: ??? Step 3: World domination!
Pretty much the entirety of Edgar Allan Poe's only full-length novel, The Narrative of Arthur Gordon Pym of Nantucket, is a long, mean-spirited practical joke at the expense of readers. In an age where exploration narratives were one of the most popular forms of literature, he published it without a label indicating it as fiction, made it completely unbelievable, and turned most of the plot into a series of anticlimactic moments, and the ending is infamously abrupt. First editions tend to have marginalia along the lines of "I don't believe a word of this!" and "Damned liar!". Despite this, the work has often been analyzed, especially the final chapter, which is also notable for being one of the earliest known examples of Cosmic Horror.
Wedge Antilles, in the X-Wing Series, loves to give straight-faced lies to his friends and peers. (Subordinates are mostly exempt.) Han Solo tends to fall for them, Leia generally sees right through him - "You're such a liar", she says once - and even Luke was fooled at least once - the Marvel Star Wars story in which Wes Janson dies has been worked around into a story Wedge tells. Wes himself similarly enjoys trolling Wedge, who plays straight man for him unless a line is crossed, as in Solo Command, which led to a truly amazing reversal.
According to the spinoff book for The Thick of It, The Missing DoSAC Files, Malcolm Tucker is one. Apparently, when he can't sleep, he spends all his time thinking about how much various public figures annoy him, and vents his anger by drafting abusive emails to them, which his assistant edits down into heavily Bowdlerized, friendly versions - such as Malcolm's suggestion to Bono that he go and contract AIDS getting turned into a 'radical new proposal to increase AIDS awareness', and 'fuck off and die, pal' becoming 'have a fucking great day, pal'. However, when he sends the draft of a foul email to Sky News presenter Adam Boulton to Sam, it gets returned saying that there's 'nothing wrong with it', so Malcolm sends it as is.
Kaitou from Kamen Rider Decade is a mild example in that he only purposely trolls one particular person, but every time he's onscreen with Tsukasa, he takes the opportunity to mess with him and try to ruin his plans, complete with a grin on his face. As the show continues, he eventually becomes a bit more friendly, but then in the finale, after tearful confessions of camaraderie, he beautifully comes full circle when he unexpectedly shoots Tsukasa in the face for a season-ending cliffhanger.
Best exemplified by his cameo in the Kamen Rider Den-O movie The Onigashima Battleship (where he trolls someone else for once), which is nothing more than teleporting in, siccing copies of past Riders on the DenLiner crew, and teleporting out.
Thanks to Villain Decay, Narutaki ended up as this: after the first few episodes, he did very little other than taunt Decade every time something went wrong.
The Top Gear America Used Car Challenge special essentially required them to troll Alabama by writing slogans on each other's cars.
Rajesh Koothrappali from The Big Bang Theory is not above mocking even his own friends when the opportunity for it comes.
None of them are above that, really.
Sherlock Holmes on Elementary frequents a conspiracy theorist BBS as a hobby. The joke, of course, is that the hobby is studying the other forumgoers, up to and including making up conspiracies out of whole cloth.
Sherlock: I adore them. As one does a barmy uncle, or a pet that can't stop running into walls.
Joe Carroll manages to escape from prison, and take control of his cult. He occasionally calls Hardy on the phone, just to give him a Hannibal Lecture. It backfires in episode 12. Carroll calls Hardy, who reveals they found the cult's armory/training ground. Carroll and the other cultists quickly realize this means they're screwed.
In episode 13, Hardy manages to troll Carroll mocking him over the fact that Roderick has been arrested, and that Roderick kidnapped Joey. When Carroll tries to give a Hannibal Lecture, Hardy just hangs up on him. Carroll who does not take any of this well.
In season 3 of Babylon 5, Sheridan pulls an elaborate troll move on the League of Nonaligned Worlds in order to get them to agree to let Whitestar ships patrol their space. A classic case of reverse-psychology on a group who would normally argue the toss about anything that was for their own benefit.
The Nigerian trickster god Edshu: In one story, he walks down the road wearing a hat that is red on one side and blue on the other. When people on one side of the road ask "Who's that going by in the red hat?", they get into fights with people on the other side who insist that the hat was blue. The god takes credit for this, saying "Spreading strife is my greatest joy."
Akita Neru is a Vocaloid character created during a troll attack in the Japanese 2ch message board accusing the characters of being pointless moeblobs, and calling everybody who used the program a misogynistic freak who wanted the illusion of controlling women. She is now considered an Anthropomorphic Personification of trolls.
Warhammer 40K: the Chaos god Tzeentch can be considered this, given that he's the god of scheming, magic, and backstabbing. One example is a traitor governor asking a daemon for something that would break the Dark Angels' siege on his fortress. The daemon complies, giving him a teleport homer that allows Dark Angel Terminators to deep strike around him, quickly ending the siege.
This is usually followed by someone yelling or raging out a single word: CREEEEEED!
Asdrubael Vect of the Dark Eldar. To wit, this is a guy who once disposed of a rival archon by sending him a black hole in a giftbox. He also at one point seduced a young noblewoman and manipulated her into assassinating her own annoying father.
In one short story, he ordered a particularly resilient slave to be brought before him. He poured a glass of wine for the slave, sat him down in a comfortable chair, and began to tell him the story of how The Fall happened, and how the Dark Eldar and their way of life came to be. When the slave asked why he was doing all this, Vect explained that he simply enjoyed telling the story and that everyone in his household already knew it. Vect intentionally piqued the slave's curiosity by refusing to finish the story, and sent the slave back to the torture pits, but not before informing him that the wine he just consumed is not fit for human consumption and causes them chronic stomach cramps. What a dick.
In The Sims 3, Sims with certain traits, evil most notably, can troll forums, and derive fun from doing so.
Hazama / Terumi from BlazBlue practically revels in trolling and taunting everyone he comes across, as shown heavily on Ragna and Rachel. By Continuum Shift, he has practically trolled the entire cast because no matter what you do, even if you Astral Finish him, he always wins. He's even been named the god of trolling!
Forum Warz is an RPG where you play a troll - an Emo Kid, a Camwhore, a Hacker, or a Perma-Noob. People contact you to bring down forums by "fighting" forum threads, derailing them, and posting nonsense until you "pwn" the entire forum.
Borderlands 2 gives us Handsome Jack. He's a petty bastard who enjoys calling up the Vault Hunters to fire insults at them. Sometimes it's just childish, sometimes it's brutally twisting the knife after a Player Punch, and sometimes it's to point out that the player is really doing terribly, but it's never for any point beyond aggrandizing himself and angering the Vault Hunters.
Mass Effect 3: Javik has been described by players as the galaxy's oldest and greatest troll. Most of his dialogue consists of him claiming various ways the protheans were more advanced than the galaxy's current "primitives", and he also loves letting someone else run their mouth and then completely shutting them down in a single statement.
He's also been known to wind up his teammates with outrageous lies about how things were in his cycle, knowing they'll never be able to tell between him making something up and a genuine Prothean tradition. For instance, when observing a poker game in progress, he casually mentions a game the Protheans "used to play" involving a sort of sacrificial combat arena... which he made up out of whole cloth, since his real hobby these days is mocking primitives.
EDI is also far too fond of jokes about robotic overlords and mechanical failures for some of the crew's taste.
Zinyak of Saints Row IV is, like, troll grand master. Every single thing he does is to mock and hurt others, and he loves causing misery to people, he loves rubbing salt into the wound, and he loves pulling petty stunts simply for shits and giggles.
In arcs where she is the one who goes insane (especially Meakashi-hen), Shion Sonozaki from Higurashi: When They Cry becomes a truly horrifying troll, taking sadistic pleasure in the havoc she wrecks through phone calls and misdirection. Normally she's The Tease.
We also have Takano, who actively provokes the Hate Plague in at least 4 of the first 6 novels.
Barring Gretel, the witches of Umineko: When They Cry are all trolls of varying degrees of maliciousness, ranging from the capricious and amoral Lambadelta, all the way to full blown monster, as is the case with Bernkastel. The copius amounts of trolls gave the series the Fan Nick Name "Trolls trolling trolls trolling trolls."
The twelve trolls in Homestuck are actually major characters. They start off as simple internet trolls, but later befriend the four protagonists and act as exposition. And they are literal trolls, i.e. aliens from a different planet.
For Dolan, a badly drawn, Ax-Crazy, rapist knock off of Donald Duck, cruel emotional torment (for his own entertainment) of the other toons is the least of his terrible acts. All of this played for Black Comedy. One of his more infamous incidents involves giving Gooby a car as a gift, only to reveal that it was a stolen vehicle, resulting in Gooby being framed for grand theft auto.
Parodied in a College Humor sketch featuring an Internet troll living under a bridge. He blocks the road and shouts racist, sexist, homophobic, or just plain inflammatory comments at people until they lose their temper, at which point they get sent flying Monty Python style. The only way to defeat them is to agree with everything they say until you can get them to unironically and genuinely admit vulnerability and the need for friendship, which imposes the same fate on them as their victims.
Parodied in episode 48 of Yu-Gi-Oh! The Abridged Series, where Internet trolls abduct Téa and prepare to cook her to death with their flames ("ONE STAR! DOWN THUMB!").
in one angry letter Yahtzee of Zero Punctuation was called "A Professional Troll". To him it invokes an image of a hideous creature under a bridge handing out business cards.
Starscream of Transformers Prime often enjoys the suffering of others, one of the best examples being when he tried to destroy Bumblebee's T-Cog (the Cybertronian equivalent to an organ) after holding it hostage, then taunting Bee about it before escaping.
"Time to jet. Because I can!"
On Wander Over Yonder, Wander deals with a tiny troll who tries to get the food from a planet of sheep warriors. Every time someone responds to his insults, the troll gets bigger and more powerful. Wander figures that the way to defeat him is to ignore him, which causes him to shrink down.
Lex Luthor admits to being one in Justice League Unlimited, as least during the second season where his entire presidential campaign was a farce to get under Superman's skin.
Referred to in the introduction to David Hume's Enquiry Concerning the Principles of Morals, in which he calls people who are "entirely disingenuous, and really do not believe the opinions they defend, but engage in the controversy from affectation, from a spirit of opposition, or from a desire of showing wit and ingenuity superior to the rest of mankind" the most irksome of all types of people. Hume goes on to state that "the only way, therefore, of converting an antagonist of this kind, is to leave him to himself. For, finding that no body keeps up the controversy with him, it is probable he will, at last, of himself, from mere weariness, come over to the side of common sense and reason."