Every kid in Hunter × Hunter who isn't Gon. The Zoldyck family is made up mostly of child assassins.
Black Butler: Cute little Ciel, only 12, has made a contract with a devil to slay his enemies. He even shot a pedophile in the face with his own gun & then had his butler kill everyone in the mansion, even though he was ordered to rescue them.
Even worse than Ciel is anime-only character AloisTrancy, who seems around the same age as Ciel and acts doubly as bloodthirsty; his introductory episode has him gouging out the eye of a maid, with his fingers, in a sudden bout of fury that she looked at him, for one thing...
Eren from Attack on Titan is a heroic version. At the tender age of nine, he tracked down a gang of murderers that had kidnapped a girl (Mikasa) and proceeded to lure them in with fake tears. When the first man got down to comfort him, he stabbed the man in the throat with a knife. The second fared much worse, getting stabbed with an improvised spear, knocked to the ground, and stabbed over and over again while Eren screamed at him. When questioned about his actions, he calmly stated the criminals were simply animals that happened to look like people. His actions were ruled justified, but the officers investigating were visibly frightened by what a nine-year old boy had done.
A Justified case exists with the Colossal Titan. An 11-year old was responsible for all that death and suffering, though years later they express remorse for their actions, with it implied they didn't fully understand their actions at the time.
Elfen Lied is about a race of superpowered psychotic little mutant girls called The Diclonius. The greatest danger comes from them developing tremendous telekinetic powers at an age when they don't yet fully comprehend hurting other people and accidentally kill their families, but most that could be captured alive become highly aggressive and violent in response to the treatment they get from the people experimenting on them. And then there's Mariko, a 5 year old girl who is so powerful that she grew up inside a sealed vault and fed by tubes her whole life and unsuprisingly became completely insane. The only thing that can keep her in line are explosive implants all over her body that can be triggered remotely at any time and also require a remote confirmation code every 30 minutes or explode automatically.
The little girl assassins in Gunslinger Girl may also be viewed as Enfants Terribles, although they have been specifically brainwashed and manipulated into being that way.
Noir is a case where the Enfant Terrible gets a bit better, more or less.
Wen from the Cowboy Bebop episode Sympathy for the Devil is a killer who stopped aging in his childhood after exposure to Green Rocks.
Excel♥Saga has an example of this: Cosette◊ from the episode "Increase Ratings Week" is an eight-year-old assassin.
Cynthia from Overman King Gainer is a perfect example of this. She's an elite soldier working for her foster father, who enjoys killing and likens it to a game - except she knows she's actually committing horrible crimes. However, her father legitimately loves her and is proud of her for her deeds, so she might just be the perfect soldier.
Bleach: In her early appearances, Ririn used the trope for her facade. During one of the fights, despite everything she had brought down on them, both Ichigo and Renji make note of how hard it is to fight brats, and instead opt to attack her two minions.
Masato from Mermaid Saga who is an adorable little child who happens to be an 800 year old sociopath known for poisoning women with mermaid's flesh in hopes for the Million to One Chance (an actual one, not the type that occurs nine times out of ten) that it might make her immortal instead. The "lucky" one who doesn't die an agonizing death doesn't have it much better either, as he tends to punish them by repeatedly killing them.
Dio of JoJo's Bizarre Adventure as a child killed his father, got adopted into the rich family of the main character, made said main guy look like a bitch for all of his early childhood years, kissed the girl that the main guy liked, burned his dog in an oven, and beat him up in a boxing match. And this is all before he becomes a vampire and causes trouble for three of the main character's descendents. His primary motivation for the vampirism? The main guy beat him for having kissed his girl, so he decided to get back at him in the worst way possible.
In Part 3, the user of the Stand Death 13 is a remarkably intelligent and sadistic baby. Whenever his victims fall asleep, Death 13 pulls them into a dream of an amusement park and kills them; his Stand makes it real. Should they escape, they have no memories of the dream. Kakyoin manages to cut a warning into his arm, but is unable to convince the others. Fortunately, he figures out how to bring his own Stand into the dream...
Hansel and Gretel of Black Lagoon play this archetype well past the hilt with their childish fits of pique-turned-bloodbaths, shared multiple personalities, laundry list of psychosexual disorders, and cutely-adorned heavy machinegun. They aren't supernatural, their immunity to recoil forces aside, but they are very, very messed-up. Justified, duetotheir traumatizingfreudian excusesof a past.
Saffron, Ranma's final foe in Ranma ˝, first appears as a very nasty child with a chip on his shoulder and a kingdom of powerful, flying soldiers at his beck and call. He later acquires his true, mature form and proceeds to devastate half the countryside.
Narrowly averted with Tokito of Samurai Deeper Kyo, who is referred to as sadistic by one of the characters (who isn't a very nice guy himself, when it comes down to it) and has a lot of onscreen fun torturing people, both psychologically and physically. Eventually, though, she is redeemed.
Vino from Gash Bell is an INFANT whose demon partner is more or less Hitler mixed with Satan. And he enjoys watching him destroy the world. After his demon partner is killed one of the main characters adopts him because he's still just a baby.
The Fifth diary holder, Reisuke Houjou in Mirai Nikki, is one of these, being an adorable 4-year-old who uses his magic picture diary to adorably kill people (i.e., asks a girl to take a bath with him, and then tries to electrocute her). He has a Freudian Excuse that his parents didn't love him enough (they never "slept like a river", next to each other with him in between) due to the influence of the Omekata cult. Well, that and God is telling him to kill people. No, not as in he's Hearing Voices that he attributes to God, but as in God exists, and he's telling a four year old to kill people. And he tells that to other people, too. Because he's THAT much of a prick.
As a small and relatively cute child, Gaara of Naruto reacted to another child's rejection by trying to kill her. Hard to blame him, though, after all the Mind Screw-ings his family gave him.
Desil Galette from Gundam Age takes this trope Up to Eleven. While he's only 7 years old, he's done more than his fair share of heinous acts.
Mokuba Kaiba comes disturbingly close to this in the beginning of the Yu-Gi-Oh! manga (when he's still trying to kill Yugi), but he gets better afterwards. Seto Kaiba around the same age (flashbacks) might have been even more dangerous. He still is.
Noah Kaiba from the Virtual Nightmare Filler Arc is Seto and Mokuba's adoptive brother. Seriously injured just prior to their adoption, Noah's mind was placed in a virtual environment that preserved his conciousness but at the cost of permanently stunting his emotional growth. Despite being in his late teens or early twenties, he still appears as a child within the virtual world, and acts like one as well.
Despite not having a humanoid form, Ghost in the Shell: Stand Alone Complex's Tachikomas evoke this at times, speaking with cute, childlike voices and behaving in cute, childlike ways just before cheerfully shredding a whole platoon of infantry with a Gatling gun.
Although he may be a bit too old to count 16 year old Hajime Muroto from Gantz certainly displays the traits; he plays it out like he's a very innocent and sensitive boy, but as it turns out he's a dangerous psychopathic serial killer and rapist.
Isaac, the prepubescent mass-murderer from Eternal Sabbath. In his defense, however, he has a Freudian Excuse: He is a clone, created for the specific purpose of having a spare to dissect as soon as he reached maturity. He was raised in a vat to keep him from developing conciousness, but being telepathic, he has known of his intended purpose since infancy, and has consequently developed a rather poor opinion of human morality.
Jellal from Fairy Tail. While he's an adult at the start of the story, he convinced everyone to be his slaves as a child because they thought he was a sweet, morally righteous kid.
Justified, for no one knew that Jellal himself was possessed.
Subverted with V.V. in Code Geass: even though he looks like a little kid, he is really in his late 50’s early 60’s and has a code for immortality.
Played straight with Rolo. The kid is an assassin, who's been killing people since he was a small child. Nobody would suspect a six year old to shoot someone, but it helps that he can stop people's perception of time.
Except that he's actually a doll, that's being controlled. Lambo, or the Arcobaleno babies would fit better, even though they were adults transformed into babies.
The purest form of Majin Buu was Kid Buu, a child-like form of Buu completely lacking any sense of morality or moderation. Essentially, his first act on being cleansed of those features absorbed from past victims was to blow up Earthwhile still on it, reform himself, and start blowing up other planets while trying to find Goku and Vegeta.
Young Knives from Trigun simultaneously murders and breaks up the crew of the ship who adopted him and Vash, and in the end he leaves everyone including Vash's beloved mother figure to die on the ship after he causes it to blow up.
The Digimon Emperor alias Ken Ichijouji from Digimon Adventure 02 has elements of this, but a lot of that is because he simply doesn't realize that the Digital World is Not a Game until he is defeated and then his Digimon partner dies. Just watch his childlike glee when he creates the Dark Spiral and Chimeramon.
Loveless has Natsuo and Youji Sagan, child soldiers genetically engineered not to feel pain. Their first appearance has them killing two people and a dog, and nailing Soubi's hand to the ground just to see how he reacts. They do get better, though.
Fate Prototype gives us Manaka Sajyou, an adorable-looking 13-year-old who's also an Omnicidal Maniac who, having been connected to The Root since birth, has a total lack of empathy and a terifying quantity of power. Manaka was so innocently evil that she sacrificed her father and many local girls to the Holy Grail, and only being stabbed to death by her own Servant when she attacked her little sister Ayaka managed to stop her... until she was revived several years later.
Malice Vundabarr of Darkseid's Female Furies in The DCU.
In the comic book L.E.G.I.O.N. '94 (the "modern-day" version of the Legion of Super-Heroes), the superintelligent Lyrl Dox took control of the intergalactic police force L.E.G.I.O.N. from his father, Vril Dox (the son of Brainiac), brainwashed all but a handful of the force's members, and turned it into a fascist organization that nearly took over the universe. All of this before his second birthday.
Calvin, from Calvin and Hobbes fits most of the requirements, but he doesn't get away with it.
Jenifer, from a '70s comic by Bruce Jones. She's got some sort of preternatural power that she uses to seduce men and rob them of their logic, driving all their friends and family away and then raping them repeatedly, before forcing them to take her out in the woods and poise to kill her, paralyzing them just as another man comes along to save her and start the process all over. Naturally, as this is a comic, her hideous deformityshould be a clue◊. (Try YouTubing the TV version, as well— there's some pretty sick stuff there.)
Though said TV version plays her somewhat more morally ambiguous. Rather like a wild animal actually.
And that's not signs like lurking creepily in rooms or asking "Don't you want to play with me, Mother? We can play such wonderful games" It's signs like killing people with his bare hands without a shred of remorse in front of everyone.
As of now, it appears that this was a subversion, Oliver is very much one of the good guys. Probably.
Same goes for the Cassandra Cain Batgirl, minus the government. She was raised from birth to be the world's best assassin, to the point where her father never spoke to her so that she'd only be able to understand body language, and actually shot at her. But, after her first kill, she read the body language of someone dying and realised how horrible it is, and thus ran away from home, drifting until she found herself in Gotham City.
Similar to Cass is Batman's son Damian Wayne. Raised to be an assassin by his mom Talia, he has a major case of Raised by Wolves.
In Earth X, the new Red Skull is a 12-year old blond boy named Benny Beckley with mind control powers. Aside from attempted world conquest his crimes include killing Doc Samson (by forcing him to tear himself apart), his aunt, Normon Osborn, and others.
By the time he actually starts taking over the planet, he seems to be about 16 or 17, at least. Killing him still nearly gives Captain America a Heroic BSOD.
The Fantastic Four actually fought a villain named Enfant Terrible. Turns out it was just a misunderstood alien child. It almost destroyed Earth by pulling the sun closer to it simply because it found this glowing ball in the sky fascinating, for one.
B.P.R.D. 1946 has a demon that stayed on Earth in the guise of a little Russian girl.
Ashley in Hack/Slash. Beat his own brother to death with a toy truck for playing with it, and was smothered to death by his outraged mother with a teddy bear. Survived as a vengeful ghost murdering other children in their dreams. Then ended up possessing the teddy bear...
"All those years of dipping from the same genetic pool caused a wrinkle, a flaw in an otherwise normal family line. We set out to keep ourselves above the common man and found ourselves with a thing from the deepest pit of the Sith."
I Luv Halloween - Moochie, oh my dear motherf* cking god, Moochie!! She takes enfant terrible and ups it up to eleven (probably twelve)! She kills an obnoxious teenage couple and a young boy by bashing their heads in with a bra-slingshot, mutilates an unfortunate doctor (by applying him with electric shocks and stabbing scalpels into his thighs), pulls out teeth from dead and living alike (she was dressed as the tooth fairy), and her mile-wide pet cemetery implies no animal ever lasts more than five minutes in her care. She's also accompanied in her murderous hijinks by Spike, who is more of a creepy child than enfante terrible. All of this is for comedy.
The main cast is nearly made of enfante terrible. In one volume, after being taken in by a Christian couple, whose fear for Finch and Mr. Kitty's safety is misguidedly justified due to being in the midst of a zombie apocalypse (the kids themselves don't care what's going on as long as there's candy), they set fire to the husband's wife before escaping.
Finch (who happens to be the leader of the loosely-knit tricker-treaters) does it again when an old woman gives him an apple for candy, which he sees as unforgivable. If he doesn't get any treats, then the old woman gets "tricked", when Finch and his gang give an apple stuffed with razor blades to a town cop, whose death prompts the woman to get arrested. 
Hit-Girl from Kick-Ass. While in her civilian garb she plays off as an innocent little girl, but outside of this, due to Training from Hell from her father, she is incredibly skilled with any weapon but she favors swords and knives in particular and can slice through an army of thugs in minutes and can shatter a man's leg with a single kick. She does all of this with sadistic glee, and she's one of the good guys.
The villains of the X-Men event "Schism" are a quartet of rich 11-to-12-year-olds who have joined the Hellfire Club. They include the new Corrupt Corporate Executive of the company that manufactures sentinels, who got the position by murdering his father and blackmailing the board of directors; a Bavarian child prodigy descended from Victor Von Frankenstein; an Ax-CrazyBadass hotel heiress; and a West African Self-Made Orphan who's taken over his family's slave trading empire.
Recent X-Force villain Genocide is a ten-year-old boy. A giant, radioactive skeleton in an intimidating-looking containment suit, but a ten-year-old boy. He's got enough nuclear firepower to blow up a town, and he curbstomped Wolverine. Subverted in that he doesn't appear to be a "bad" kid, exactly, he just admires and tries to please really bad people (the followers of Apocalypse, if you must know).
Jimmy Marks aka Hybrid. The son of a human woman and a Dire Wraith. He started out as a relatively normal child, but when a bunch of his dad's old comrades showed up looking for him, they took an "interest" in the boy and taught him the ways of power and evil. Jimmy took to their lessons like a fish to water and started terrorizing his parents (even his Dire Wraith father was terrified) and killing livestock For the Evulz. Even worse, around this time Jimmy's mutation — extremely potent Psychic Powers — also manifested itself. Psychic powers, the ability to go One-Winged Angel into a super strongNigh Invulnerable monster even fouler than a typical Dire Wraith, and a penchant for evil that terrifies his own Dire Wraith father (who is a member of one of the most evil races in the Marvel universe, which is saying something) — not a good combination. In his debut he ages his own mother to death, impales his father with a pitchfork when dad tries to kill him out of remorse for letting him become a monster, engineers a Let's You and Him Fight scenario between Rom and the X-Men by pretending to be a helpless child, and tries to abduct Kitty Pride for breeding purposes. Hybrid was so evil that Rom was willing to set his weapon to "kill" when Hybrid proved too powerful to be banished to Limbo. Hybrid not only survives, but gets worse with each later appearance. In Avengers Academy, he pulls the exact same "helpless youth" act to get admitted into the Academy, all so he can enslave everyone there and make the men his soldiers and the women his breeding partners.
Ulysses Hadrian Armstrong, one of Tim Drake's recurring foes, definitely qualifies. He was a mass murderer by ten, nearly started a war by eleven, and graduated to full-on criminal mastermind by age fifteen.
In Squee, the six/seven-year-old Pepito is this... on account of being literally The Antichrist. His introductory scene has him disintegrating a child's head.
Deconstructed in the Firefly fanfic Forward, where the resident psychopathic child is a mind-controlling girl created by the Academy. The deconstruction comes from the fact that the child in question is impulsive and irrational, going on violent rampages whenever she thinks she's encountered someone connected to her torture. This ultimately leads to both the Hands of Blue and the crew of Serenity tracking her down, by essentially following the trail of corpses.
Brox in With Strings Attached was an elven woman of unknown age who was murdered and then reborn as a five-year-old boy. Given her taste for sadistic jokes (that's why she was murdered), considerable knowledge, and willingness to step on people to get what she wants, she qualifies.
In Attack Of Giygas, we have Porky and Ashley. Ashley ends up turning Lucas into one.
My Little Wesker expands on Wesker's history quite a bit. He was an advocate of eugenics and Blue and Orange Morality at an early age, and at thirteen years, Wesker successfully pulled off a murder of a 16 year-old bully after he decided he was a threat to his continued development.
Anna is shown to be this in a parody of "Do You Want to Build a Snowman?" called "Will You Help Me Hide a Body?" At the beginning of the song, she asks her sister Elsa to help her hide someone's body "before he can decay" singing that "it doesn't have to be in one piece". She's five years old at that point. Her singing about throwing someone down a well, digging up graves and running out of space to hide bodies in caves when she's nine shows that she doesn't get better.
In the Gensokyo 20 XX Series, Yukari was this is as child, along with being a Child Prodigy, being able to manipulate other youkai before she was out of babyhood (she was three when she started) and often manipulated them into attacking villages and playing the Wounded Gazelle Gambit.
In The Tantrum Phase and it's sequel, we have this with Chen. a rather tame example, who, in the former, throws tantrums and behaves horribly, even causing Ran and Yukari injury, and, then, in the latter, she tricks Marisa and both make trouble for Ran.
City of God features Lil' Zé, who cemented his position as a monster when he was still only a little kid, and casually walked into a brothel and gleefully killed everyone inside, just to satisfy his bloodlust.
Damien from The Omen. Your chances of being a decent kid aren't exactly great when you're the the antichrist...
The kids from Village of the Damned. In the original film, a boy hits one of the kids with a basketball sort-of on accident. He later drowns.
The Bad Seed, which was a 1956 theatrical film (in which the Enfant Terrible was named Rhoda) and a TV movie in 1985 (where she was named Rachel).
It's Alive is about a malevolent baby. Within minutes of its own birth, it kills everyone in the room except mom.
David Cronenberg's The Brood has a woman who has developed the ability to parthenogenetically give birth to mutant children who represent all her negative emotions, and exact bloody vengeance on anyone who has done her wrong, whether real or imagined.
The unstoppable Paperboy from Better Off Dead pursues Lane throughout the movie, demanding the two dollars Lane's family owes him. He also duplicates himself/recruits an army of identical helpers, and survives falls that should kill a human.
The recent underrated film Orphan stars a serial killing orphan, or so it would first appear. She's later found out to be an escaped 33-year-old Estonian mental patient with a pituitary disorder and a lot of makeup. It's a lot smarter than it sounds on paper.
Samara in the remake of The Ring was aged down from 40 or so in Ringu to about 12. Oh, and she can drive you insane or kill you with her mind. Even after death.
The title character in Joshua was possibly one of these. But it is also possible that everything that happened was just happenstance and misunderstandings. In the final scene, it is revealed that yes, he had arranged the destruction of his entire family, just so he could go live with his favorite uncle.
The title character in Mikey is one of these as he is a seemingly innocent child who is actually a psychopath who murders his foster families or anyone else who makes him really angry.
Men In Black lampshades this in a scene involving a shooting gallery filled with aliens. While the other MIB applicants shoot at the aliens (who're actually just working out, sneezing, etc.), Will Smith's character shoots the lone civilian, an "...eight-year-old white girl, middle of the ghetto, bunch of monsters, this time of night with quantum physics books? She about to start some shit...".
In Bloody Birthday three children, Debbie, Curtis, and Steven have no consciences and murder anyone either because they feel like it or feel they have wronged them; it was foretold in the movie that anyone born during an eclipse would cause them to behave that way.
In Halloween (1978), Michael Myers is six years old at the time of his first murder. Even more so in the remake, with Deborah finding the notion of Michael killing animals unbelievable. Until he graduates to people.
Oogie Boogie's hench-children Lock, Shock and Barrel from The Nightmare Before Christmas. While discussing their objective of kidnapping Santa Claus, the possibility of killing him in various ways also comes up.
While not cute per se, Winchell in North certainly is terrible, a Smug Snake really. This preteen school newspaper editor encourages the hero's search for new parents and publicizes it. While North's out searching, Winchell spearheads a powerful movement to make all parents subservient to their children. When he realizes that North choosing to return to his real parents would spoil things, he tricks North into thinking they no longer love him — then sends assassins (adults) out to kill him.
In Who Can Kill a Child?, children on an island of the coast of Spain for no particular reason start murdering all the adults until only they remain and they go off to recruit other children for their killing spree. The only explanation for their psychotic behavior was that one of the initial survivors claimed that a virus that only affected the children may be causing them to act like that.
Jody Mitchell from Daddy's Girl is a psychopathic little girl who kills anyone who gets in the way of separating her from her beloved father.
Claudia from Interview with the Vampire is a little girl who was turned into a vampire by Lesat to give Louis a surrogate child. Upon becoming a vampire, she is a blood-thirsty murderous child who lures people to her by pretending to be lost so she can kill and feed off them.
Parodied in Mafia with Chucky, a psychotic little boy who is a parody of Chucky from Child's Play; he murders his grandfather with bug spray so that he can steal his wallet and his father can become the new godfather, he is later seen torturing animals and stabbing dolls, and he gets his comeuppance when he receives a package of tiny dinosaurs which eat him alive.
The Children semi-combines this with Zombie Apocalypse, as an unknown sickness affecting only children turns them into murderous sociopaths who are very good at faking innocence. Then it turns out teens are affected too; it just takes them longer to turn...
The main character of The Tin Drum is more of a sociopath than a psychopath but he's still terrible nonetheless.
At the end of Identity, Timmy York turns out to be the one who committed all of the murders at the motel and kills his mom in her deathbed.
Poked fun at throughout Son of the Mask. Alvey isn't an evil baby, but he just feels that his own father doesn't care about him, so he uses his Reality Warping abilities to attempt to drive him insane.
The spoiled, precocious, mean-spirited brat in the film version of The Day of the Locust. He eventually takes it too far and pays with his life for it.
Played for comedy in Problem Child. Junior and later Trixie, his arch-rival and later best friend and stepsister.
Gaia in the Gone series, so very much. Within the four hours from her birth to the end of Fear, she manages to torture Penny after she accidentally drops her and then laugh at the scene; laugh, once more, at seeing terrified children walk into a fire due to Penny's visions; and forcing her mother to relieve her horrible memories of eating Panda. And that's not all... She also attempts to kill both her own father, Caine, by trying to crush him against the FAYZ barrier and Sam, by trying to rip him apart via telekinesis. Thankfully, she doesn't succeed in any of the two cases.
The two children, Miles and Flora, in Henry James's The Turn of the Screw. Although there is some speculation as to whether or not they really were Enfants Terrible or whether the narrator was insane...
Florence in Florence and Giles, John Harding's version of The Turn of the Screw, turns out to be very much this, given her habit of murdering her governesses.
Acheri, from Hell's Children, by Andrew Boland, is a little girl who sadistically kills many people through the course of the story.
Josephine in Agatha Christie's Crooked House. She's responsible for a series of murders starting with her grandfather.
Gwendolen Chant from Diana Wynne Jones's Charmed Life may look like a china doll, but for her vindictive nature and her mistreatment of her brother Cat.
An even creepier example would be "Zero Hour", in which every child in the world is convinced by an alien race to set things up to let them invade Earth and kill all of the adults. And they agree because they are promised later bedtimes, no baths, and all the TV they want. And it ends with the main character's daughter leading a group of aliens straight to her parents, while calling to them as she searches the house.
Clark Fries in Robert A. Heinlein's Podkayne of Mars. (Although he's on the way to reform at the end of the book.)
Amity Adora ("Amma") Crellin in Gillian Flynn's Sharp Objects; charming, childlike, spoiled, vicious, manipulative, morbid, sexually precocious, and, incidentally, a triple murderer by the age of thirteen.
Tara Webster, Michael's little sister from the Goosebumps book The Cuckoo Clock of Doom appears to gain a sadistic pleasure from making her brother's life difficult, and displays no redeeming qualities at all. It appears that she's ALWAYS been like this, to the point that she would make things difficult for her brother even when she could barely speak. At the end of the book, it's hard to feel bad about Tara's existence being erased.
And the Ass Saw the Angel (a book written by Nick Cave, whose musical career provides the page quote) has the character of Beth. The Unreliable Narrator can't quite make up his mind whether she's actually a demonic witch or not.
Tad Williams's The War of the Flowers just straight out calls it The Terrible Child. Everyone knows he's pure evil though. That was kind of the point of creating him in fact.
Lord Hellebore: That child is an abomination.
Remover of Inconvenient Obstacles: Then you have achieved your purpose, my lord.
Angel from Maximum Ride, who has a very innocent appearance.....but can read and influence minds, and can communicate with fish...but mostly sharks. She also tries to take over the flock from time to time.
The Royally Screwed Up Joffrey Baratheon. King at the age of thirteen, his favourite activities include ordering executions, having men fight to the death, and shooting starving peasants with a crossbow. He also cut open a pregnant cat as a child.
At age twelve Gregor Clegane burned off half of his younger brother Sandor's face when the latter dared to play with one of Gregor's toys, a toy that Gregor did not want anymore.
Her Thumbleness in Dragon and Slave likes to select slaves to be entertained by, in various ways. Scaled ones get painted on or in one case carved. Others mostly get beat up.
Princess Violet from The Sword of Truth. A spoiled princess who orders to chop off heads a-la the Red Queen. A few books later, she is taught to cast curses through magical drawings. Cut to a Little Miss Badass coming to visit; Well... Who's that on the drawing? Oh, that's right, it's me. (draws a few lines) Well, Violet, now it's you. Cut to Sound-Only Death.
In The Death Gate Cycle has one whose role spans two books. Bane is the son of a mysteriarch, who had an Evil Plan to take over the human race in Arianus. He orchestrated this by switching the king and queen's legitimate child with his own son. The kid was kept safe from harm with sheer looks, and eventually gets his comeuppance when he tries to kill the king. His previous mother then strangles him with magic, in a sort of parallel to the fate of the king and queen's original son, who suffocated from lack of air in the higher reaches of Arianus.
Dune: Alia, who is also the Creepy Child poster girl. She is born fully aware, and kills her grandfather at the age of two. And laughs. In fairness, he was the Big Bad.
Lord of the Flies is all about how a lack of immediate consequences for their actions brings out the worst in a group of castaway children.
V.J. in Robin Cook's biotechnological thriller Mutation — a boy whose superior intellect (and perhaps his angelic good looks as well) is the result of his father's genetic tinkering. He however, has no sense of morality, commits a series of high-tech murders, and uses underhand means to finance his own biotechological research.
In the Andrew Vachss Burke book Mask Market, Burke found that Beryl Preston, one of the children he had been tasked with retrieving, turned out to have been playing both her parents and her "captor". In the present day, he finds out that, far from growing out of it, she has only gotten worse.
The Big Bad of the Tunnels series is one: Rebecca Burrows, the protagonist's 12-year-old little sister.
Euphemia in My Godawful Life, a parody of Misery Lit: a foul mouthed girl genius who turned her former foster parents into spiders and who has driven many of her carers to suicide. She also hints that she fakes her Asperger's Syndrome in order to avoid being locked away, and may have murdered her father (a paedophile) and his friends.
Markie, a toddler who makes life miserable for Skeeve and his associates. Somewhat subverted in that she is actually The Ax, and mature for her race. She was hired to ruin Skeeve's reputation.
The protagonist of The Wasp Factory by Iain Banks is a 16-year-old boy who has killed three children and numerous small animals, the children at a very young age.
Haunted: Brandon Whittier is a thirteen-year-old cursed with progeria, a disease which causes its victims to age at seven times the normal rate, becoming Younger than They Look. He tricks the female volunteers at the nursing home where he works into thinking he's eighteen, begging them not to let him die a virgin, and then threatening to report them for child molestation unless they give him ten thousand dollars.
Mordred Deschain, The Little Red King, from The Dark Tower series. At least two characters fall victim to this evil baby within hours of his birth.
The In Death series: Rayleen Straffo is revealed to be this in Innocent in Death. She is The Sociopath who killed her own baby brother, two male teachers, an old woman in a nursing home, and also tried to kill her own mother. There is no Freudian Excuse for this child.
Leck in Fire/Graceling. Guess it comes with being able to put thoughts into people's heads...but a lot of people are damaged or killed thanks to him.
The Bernard Taylor novel The Godsend tells the story of an English family of six who take in an abandoned baby. As the little girl grows up, she murders the other children, one by one, so she can get all of the parents' attention. The father eventually figures out his adopted daughter is evil, but does not succeed in stopping her.
Eppon in Galaxy of Fear is somewhere between this and Goo Goo Godlike until he grows up within hours. He's cute and loving and kills people by absorbing them as soon as there's no one but the victim around to see, but seems to have a soft spot for Zak and Tash.
Rhys in The Granite Shield, by Fiona Patton, is a royal bastard touched by the Gods, with consequently very different priorities than the normal people around him. When he fights at a castle siege at age twelve, he nonchalantly talks his younger brother into climbing up the privy shaft together in the middle of the night, then sneaks into the kitchen and contaminates the commanders' goblets with a horrid disease. Within weeks, the siege is over. His brother later talks a fortress into surrendering by telling the harrowing story.
In the second Eisenhorn book, during the Thracian Atrocity, Eisenhorn comes across a child that he thought was in danger. Said child turned out to be an Alpha Class level pskyer, who managed kill a space marine before it was finally defeated.
In the Firebird Trilogy, Phoena was a wastelingnote third child, and thus required to commit suicide once her oldest sister had two children until the age of six, when her sister Lintess died. Their sister Firebird privately suspected that Phoena had murdered Lintess, but could never prove it. Certainly Phoena has always been "unswervingly selfish".
This trope is deconstructed in Funny Business. As a toddler, Jeannette was like this, because toddlers don't comprehend that other people have feelings and can be hurt. Once she grows out of it, she suffers a massive Heroic BSOD which continues to the time of the main story.
Virginia in Alex Grecian's The Black Country, who turns out to be the one responsible for murdering her half-brother Oliver. The reveal results in Offing the Offspring, followed by Driven to Suicide.
Live Action TV
The various Law & Order series like to play with these, and especially emphasizes the handwringing "what can we do, the legal system can't handle such a monster without exploiting innocent kids!" response.
An 8 year old girl who has ISSUES with the male gender: she would rip out the eyes out of males in pictures, poisoned a cat just to watch it die (and doesn't flinch when the Psychiatrist yell at her for it), scared her nanny to do her bidding... and murdered a younger boy and stuffed a battery in his mouth. And doesn't have, and likely is completely incapable of feeling, a shred of remorse. She gets off, based in part on the argument that she's too young to fully understand death, and the last shot has her coolly regarding another young boy, with the implication that more death is in her future.
There was also a Law & Order: Special Victims Unit episode with a set of Creepy Twins, one male and one female. It was revealed that the girl was actually born male, but trussed up in long hair and skirts as part of an award-winning, cold blooded psych study. One twin murders the doctor who is responsible, but as the two suspects are impossible to differentiate from each other, it leaves the detectives at an impasse.
And yet another SVU episode featured a sicko kid who murdered a "friend" at his own birthday part, and liked to mutilated himself to scare other kids. When it was realized he wouldn't get charged as an adult the murdered boy's father shot him.
Interestingly enough, the end of the episode seems to paint the father as the real monster here, revealing that he intended to shoot the kid even before the thing about being charged as an adult was revealed, as he, a psychiatrist, believes that mentally ill people who can't be cured should be killed.
The difference between the Father and the boy was that The boy killed out of enjoyment of it and would continue to do so once he was out of juvi, while The Father will never take another life, and knew the consequences of allowing the boy to live.
There's also another episode of SVU which revolved around a harassed boy that gunned down members of the school's basketball team, then tried to commit suicide and missed then blamed an alter ego named "Zoltar" for it. He gets better though, as the unit gives him the opportunity to go to a mental hospital to cure his disorder.
Another episode of SVU looks like it is playing this trope straight (kid comes to school and shoots another kid on the playground) but it is actually subverted (he was shooting at the gangsters behind the fence that have been threatening him).
The episode "Born Psychopath" revolves around Henry, a manipulative 10-year old boy who has homicidal tendencies whenever he doesn't get what he wants, to the point where he locks his mother in the laundry room, ties his sister to a bed while nearly lighting his family's apartment on fire, ties a neighbor's kid to a chair in a closet, drowns his neighbor's dog, takes another child hostage inside a playroom, and shoots Amaro in the abdomen. Fortunately, Amaro was wearing a bulletproof vest.
CSI: Crime Scene Investigation had Hannah West, a 12-year-old child prodigy who successfully got her brother released by planting evidence to implicate herself and then turned into a Yandere when her brother paid more attention to his girlfriend than to her.
Then there were the little bundles of horror that were the killers in "Bad Words" and "Cats in the Cradle." The killer in "Go To Hell" also had an early-teens accomplice who was arguably an even worse person than he wasnote He was a pedophile preying on her, but she used that to her advantage by promising to be with him if he killed her family; still, she's young enough that we can still feel for her when she herself is murdered by The Fundamentalist. Oddly enough, subverted with the killer in "Gentle Gentle," the actual youngest perp seen on the show... who was too young to know what he was doing.
The role played by (sigh) Justin Bieber as Jason McCann. This may be one of the worst ones on this list (even though he's not taken seriously because he seems "girly") because instead of sadistic sadism, he cunningly plans, kills three police officers, and does this all by trying to bomb the living shit out of them! It is however worth it in the end when he gets shot to death by the authorities after shooting one of the officers
Also from Buffy the Vampire Slayer are Hansel and Gretel, a single demon that takes the form of two dead (and later living) children apparently murdered by occult forces to cause the mothers of Sunnydale to try to kill all the other supernatural elements. Under their influence, Buffy's mom, Willow's mom, and several others attempt to burn Buffy, Willow, and Amy at the stake. It doesn't work so well when Hansel and Gretel are turned back into a big ugly demon though. Apparently the original story of Hansel and Gretel was based upon an earlier more successful attempt by the same demon in Germany.
Angel had an episode where a little boy was possessed by a demon and doing horrible things. They exorcise the demon, but find out from the demon (who begged to be killed) that it was the boy himself who didn't have a soul to begin with, and that the demon had been trapped.
To be fair, the demon would have done the same kinds of things if it had been in control. What had him scared was that while demons like him do evil things for the sake of evil, the boy's mind was utterly blank, even as he did things like try to kill his sister on a whim.
Really even saying he was cold inside is giving the kid too much credit based on what the demon said. The demon described it as being trapped inside a total emptiness and that even though he had been corrupting humans since "before [they] had words to name [him]", it was the most maddening thing he had ever known, driving him to take control of the boy's body at night to attempt suicide.
The X-Files also gave us Charlie and Micheal from "The Calusari." Michael is the ghost Charlie's twin brother, who died at birth, and is killing many of the people Charlie is close to, since the the mother never performed an exorcism
The Millennium episode "Monster" concerns a demonic lil' tyke who frames her daycare provider for abuse, then does the same to Frank when he investigates.
In Remembrance of the Daleks, a girl is possessed by the Dalek battle computer.
In "Human Nature" and "The Family of Blood", another girl is overwhelmed by the gaseous entity 'Sister of Mine', taking over her lifespan. The same musical theme is used in both cases.
The trope is also toyed with in "Closing Time". The Doctor, who claims to be able to speak baby, tells Craig that his infant son Alfy prefers to be called "Stormageddon, Dark Lord of All." There's no indication that he's an actual evil baby, though.
It also had one as a Monster of the Week way back in Season 1; in the episode Provenance, a haunted family portrait killed anyone who owned it. It turned out to be possessed by the spirit of a psychotic little girl who killed not only her own family, but then the family who adopted her. The girl also died, but her spirit haunted the painting.
Supernatural loves this trope. Enfant Terribles, usually ghosts, possessed or just plain crazy, turn up in Dead in the Water, The Benders, Playthings, All Hell Breaks Loose, The Kids Are Alright, Bedtime Stories, Are You There, God? It's Me, Dean Winchester and Family Remains. Did I miss any?
Little sister Megan on Drake & Josh is an evil, conniving brat to the titular characters, but acts sweet and adorable to everyone else.
Brat doesn't really cover it. She's a pre-teen super villain. She's got a hidden high-tech lair in her bedroom disguised as girly toys and pictures. Seriously.
Madison in the the "Everything Nice" episode of The Inside.
Parodied in Only Fools and Horses — Rodney is convinced that his nephew Damien (the name is not coincidental) is one of these, and acts as if he's with the Anti-Christ anytime he's in the same room as him. The boy's just a normal child, but try telling Rodney that.
One particular scene highlights this; Damien wants to show off a conjuring trick he's learnt, and chooses Rodney to show it to. From Damien's point of view, he's just happily playing with his uncle. Rodney, however, looks as if he's being forced to participate in some kind of satanic ritual.
From the episode The Twilight Zone "It's a Good Life": "No comment here, no comment at all. We only wanted to introduce you to one of our very special citizens, little Anthony Fremont, age 6, who lives in a village called Peaksville in a place that used to be Ohio. And if by some strange chance you should run across him, you had best think only good thoughts. Anything less than that is handled at your own risk, because if you do meet Anthony you can be sure of one thing: you have entered The Twilight Zone."
In the original short story, he's only three, which means he's not so much evil as incapable of understanding that it's wrong to hurt others, the more so as nobody's ever had the nerve to tell him "no".
The sequel episode gives us Anthony's daughter Audrey, who seems better than he is (she openly admits to hating him when she sends her friend's father to the cornfield) but towards the end of the episode quickly and remorselessly sends the entire rest of the population of the town, even Anthony's mother, to the cornfield. Because they all thought bad thoughts about her and her father. "We don't need them daddy! We don't need anyone!" But then she does bring everything back when he says he's lonely, so yeah. The episode ends with Anthony cowed by the knowledge that Audrey is even more powerful than him since she can wish things to the cornfield and bring them back while he can only send them away.
In A Nice Place to Visit, Rocky had led a street gang while in grade school.
In Desperate Housewives, Kayla, Tom Scavo's illegitimate daughter became, this. At first she was just cold to Lynette, who her conniving mother Nora had told her wanted to take her (Kayla) away from her (Nora), which was true, but only to get her away from Nora's crazy. But then the next season, Kayla does increasingly outrageous things, from convincing her half-brothers to jump off the roof to claiming Lynette abuses her, all in the name of getting Lynette out of the picture so she could stay with her father.
Similarly, in the 1998 series of the same name, the first thing we see of Mordred is him, as a young toddler, picking up a knife and throwing it across the room at a guest. According to his mother, it's just his way of demanding attention.
In the episode "The Boogeyman" it's revealed the killer was a twelve year-old boy who would lure other children into the woods, then beat them to death with an aluminum baseball bat.
"A Shade of Gray" reveals the (main) killer was a child sociopath who had murdered his younger brother by cramming model plane parts down his throat after his younger brother broke the model plane by accident. He also killed the family's puppy.
The episode "Safe Haven" had a 13-year old sociopath on a cross-country killing spree, slaughtering entire families by gaining their trust with his seeming innocence in order to get access to their homes.
In Power Rangers Operation Overdrive (made and taking place in 2007), Thrax, son of Rita and Zedd, spent a few days as The Man Behind the Man in charge of that season's warring factions, and proved a villain powerful and skilled enough to disable the entire incumbent Ranger team. Word of God says he was born between Zeo and Turbo (that's the reason Zedd and Rita didn't take Big Badhood back like the Zeo finale implied they would, they were raising him), making him just ten years old at the time.
Luckily, he later makes a Heel-Face Turn and turns out to be Smallville Superboy
Malcolm in the Middle has a few: Francis, the eldest, did several horrible things as a toddler, and ended up lighting his teddy bear on fire (and the fact that he poured lighter fluid on it implies that it was deliberate), and it is heavily implied that Francis' final action was the reason why Lois ended up having to become such a strict mother. Reese kicked his mother hard enough to force her to go into labor several hours before the scheduled time for labor, and then kicked the doctor as he was born. Probably the worst offender was Jamie, who after being born, often frames his brothers for things he did, and when under the influence of soda, actually attempts to murder Lois by shoving a shelf onto her. Suffice to say, Francis and Reese really don't grow to become any saner into adulthood.
The apparent physical age of immortals from Highlander is fixed at the age of their first death; two episodes featured an evil immortal who had undergone this at seven years of age.
A centuries old Immortal in the body of a teenager, Kenny has survived for hundreds of years by playing on his youthful appearance and the sympathies of older kind-hearted Immortals for protection since he's at a severe physical disadvantage. Then he takes their heads when their back is turned. Amanda especially grabs the Idiot Ball where Kenny is concerned, since she was the Immortal who originally discovered him (prior to his Face-Heel Turn when he really was an innocent boy) and refuses to believe Duncan's warnings, even though MacLeod already had several prior encounters with him.
Forever Knight Lacroix's daughter-turned-sire,Divia,who he staked after she tried to seduce him but who came back and started killing his friends in the modern day.
The previous page-quote, "The Curse of Millhaven", by Nick Cave and his aptly-named backup band, The Bad Seeds, is about a 14-year old mass murderer, who drowns the neighbour's kid, burns down property, decapitates the handyman (with the circular saw in his garden shed), stabs an old lady to death, and is directly responsible for a class of children drowning, but did notCrucify the Dog. Nothing odd about the mass murdering, especially on an album like Murder Ballads, but it just goes to show- even a cute, blonde-haired small town girl can be evil to the core.
The band references two earlier examples: Nick Cave and the Bad Seeds have an album called "The Good Son".
Independent L.A.-based band Creature Feature has a song called "Such Horrible Things," which follows the birth, life, various horrors, and final institutionalization of an unnamed psychotic individual, who's deeds include, but aren't limited to:
Pouring super glue into his father's hair at the age of two.
Attempting to leave a "friend" in the woods to die at age six.
Burning down his family's house at the age of eight because he hated the color of the place.
And at fourteen "there was that one time" which, on the actual song, is conveyed only by a musical interlude and lots of screaming.
Edge of Sanity's "Crimson II" is about a demonic child born to a nun who read a cursed book. The child can read minds and kill victims by projecting "images too hideous for human minds to bear".
We've got a sense of humor but we haven't any cares. Oh how I had to laugh when we kicked granny down the stairs. On our tippy-toes we crept right up where she was standing, And the tears ran down my face as she lay bleeding on the landing. And when we called out "Mama! Quick! We think our granny's dying!" And when Mama came a-running we pretended to be crying. We said "We saw her trip"; they couldn't tell that we were lying And since Granny was unconscious she was doing no denying! Ha Ha Ha Ha Ha Ha Ha Ha Ha Ha Ha Ha Ha.
And to top that, "The Cat Is Dead" by The Mystic Knights of the Oingo Boingo (the predecessor group to the better-known Oingo Boingo), gleefully delineates the actions of a trio of psychopathic children:
The cat is dead, the cat is dead, And Mikey too, and Uncle Fred Expiring oh so suddenly while sipping down some tea. The tea was hot, the tea was nice With strychnine and a little spice To cover up the funny taste Of our conspiracy. When Grandpa saw what we had done He went straight for his hunting gun. But we were quick, we stole the clip; The rest is history. To make sure Grandma wouldn't flee We gave her a lobotomy And now she's just as happy as can be.
"That's My Boy" by Stan Freberg, a smarmy Parental Love Song about a son whose most endearing habits include setting the mailman on fire and pouring acid on the rug.
Older Than Feudalism: the Greeks had Zeus, whose mother saved him from being eaten by his father Cronos (trying to avoid Karmic Death for overthrowing his father); Zeus then served up some nasty poison to Cronos while posing as a servant; his son Hermes tops this by stealing a herd of Apollo's cattle almost as soon as he was born and then sweet-talking his way out of being punished for said theft (and, on top of that, killing one of the cattle as a sacrifice to himself).
The Chinese god of battles Ne Zha (pronounced Nataku in Japan) not only was born in an egg-like sac that took three years to "hatch" (finally Dad got fed up and attacked the thing with a sword, after which Ne Zha burst out), he also brought the wrath of the Sea King down on him when he accidentally killed a young prince when said prince went topside to give him grief about his destructive playing habits.
Episode 150 of the radio-show-turned-podcast Comedy Bang Bang, an episode which is unique in that it has both guests playing characters, has Bobby Moynihan hilariously playing a young orphan named Fourvel. Fourvel at first appears to be an innocent fan of Andrew Lloyd Webber (played by Paul F Tompkins), but quickly reveals that he's actually a murderous psychopath who is prone to getting "a little stabby".
The Holy Terror from the Big Finish Doctor Who audio of the same name. The child in question is a five year-old boy who has been imprisoned in a dungeon ever since he was born so he would grow up untainted by the world, divine and all powerful. Oh, he's all powerful alright.
Do you think I'll be the best torturer in the world? Do you think my father will be proud of me? WELL? WILL MY DADDY BE PROUD?
Tabletop RP Gs
In World of Darkness: Innocents, where the player characters are children, a character who hits zero on the Karma Meter becomes one of these. They aren't necessarily evil, but they have absolutely no empathy or conscience. As the book puts it, they'll probably end up institutionalized.
The New World of Darkness game Changeling: The Lost includes Fetchspawn, one of the options for the children of the fake people that the Gentry leave to replace Changelings. They have no Karma Meter. They have no empathy. Most people just assume they're autistic because of their total inability to relate to other human beings on an emotional or social level. They tend to kill things just because. This is all exacerbated by the fact that their touch automatically opens all doors, springs all locks, they cannot be bound or imprisoned, and people tend to ignore them, so they're able to slip around without notice. They are immune to Changeling superpowers, and their touch drains the Lost of magical energies. Oh...and did we mention that at the age of 21 they get sucked back into the Hedge, likely to become Gentry themselves?
In Mage: The Ascension, widderslaint were reincarnated Nephandi (mages who were evil because of their malformed souls, or whose souls were malformed by their evil). Even before their awakening as mages, they're irredeemably evil (the fiction introducing them describes an infant crawling into his twin brother's crib to strangle him to death, then falling asleep hugging the corpse like a teddy bear).
The Dungeons & Dragons 3.5 manual Heroes of Horror, incidentally, introduces a template called Unholy Scion, which is a creature whose soul has been substituted with a demonic/devilish/evil essence while still in the womb. Already at birth, the resulting child holds his/her/its own mother in thrall through evil power alone, and gains terribly destructive powers as it grows in Hit Dice. Oh, and it is so evil that its merest touch can grievously harm good-aligned characters.
There is a sequel to Cyberpunk 2020 called Cyber Generation, where the Edgerunners of CP2020 have mostly been either killed or grown complacent by buying cushy corpjobs with retirement, leaving the rebellion to their children. It's very possible to play this kind of character in this game, especially if your character is a type of YoGanger referred as a Beaver Brat. Beaver Brats mostly come from suburban areas and specialize in talking their way out of trouble by convincing adults that there has been a big misunderstanding and they couldn't possibly have done it.
Mary Tilford from The Children's Hour is a malicious girl who tells a lie about her two teachers just to get out of going to school. She then blackmails another girl into saying she started the lie, pull's the other girl's arm out of her socket (albeit accidentally), and steals several jewelries.
Kerchak falsely believes this of a young Tarzan in the stage version of the 1999 Disney film, when he comes upon the child fashioning a spear.
In the 2013 adaptation of Charlie and the Chocolate Factory, Mike Teavee is upgraded from the Spoiled Brat of the book to this. The reason his mother allows electronic media to babysit him is because it's the only thing that he enjoys that doesn't also indulge his destructive tendencies — setting a cat on fire, stealing a German tank, etc. "[T]he authorities request/That little Mike not leave the house", she explains, and all the adults in his life seemed too cowed by what he might do to them to attempt more than token efforts to discipline him. Poor Mrs. Teavee has become a Type A Stepford Smiler in response to all this. And unlike in other versions, she's quite happy when her son winds up but a few inches tall...
One of the members of the Dark Brotherhood in Skyrim appears to be a young girl, but is actually a fairly old vampire who was bitten and turned back when she was just a child. She is known to boast about how her innocent appearance makes killing easier.
Mary from Ib is a rather powerful supernatural example of this trope. However, partially subverted in that she's actually a 400+ year old painting who was created by the artist Guarterna.
Played straight in Metal Gear Solid: Portable Ops, with a very, very deadly child soldier known as The Frank Hunter to the mercenary community, because of his habit of approaching his targets with nothing except a knife and an innocent cuteness that made it impossible for them to strike back. He later ends up becoming Null, and Gray Fox, who is the Cyborg Ninja. Whew.
Liquid might come very close to this trope, assuming the Official Missions Handbook's claim that Liquid participated as a mercenary at the age of 13 is canon. In either case, Solidus comes the closest, seeing how he was implied to have led a unit in the Liberian Civil War as a teenager (although in his case, everyone wasn't an adult).
Raiden, who was raised to be a child soldier after his parents were murdered by Solidus, and placed in a unit of young boys that fought in the Liberian civil war. His nicknames were 'Jack the Ripper' and 'White Devil'.
Kira Daidouji, an 11-year old child genius who got fed up with having to return to Japan to mix in with the "normal kids". Her research on ether science has allowed her to create a large watery blob, whose body she uses for combat. Sounds normal at first, but when you consider she also wants to use that blob to help her Take Over the World, you're in completely different territory.
Lieselotte Achenbach is a ten-year-old assassin who carries around the spirit of her dead older sister in a puppet with no legs. Creepy!
Independent adventure game Emily Enough features this as the player character: Emily decides, at her birthday, to slaughter her family. The rest of the game features her trying to get out of the asylum she ends up in. Emily herself seems quite comfortable with her new profession, too.
Occurs occasionally in Super Robot Wars, but most prominent with the leader of the Inspectors: Wendolo. Killed his own brother and threw away his Quirky Miniboss Squad without shedding a tear, felt no shame in desiring to enslave humanity and reduce them to mindless killing machines (Incidentally the exact same plan as that of the Balmarians, who were the sworn enemy of the Inspectors' superiors), refer to humanity as nothing more than a cancer that needed to be eradicated, and doing it all with a smug grin on his face.
In the same quest (and the entire game with a proper console code/mod) you can be one yourself. The biggest example perhaps being to kill Timmy's parents, and then brag to him about it.
There's also the in-world, pre-war Urban Legend of the "Pint-Sized Slasher" An unknown kid in a clown mask with a kitchen knife, going around killing adults. Whether he exists or not is up for debate, but Betty (Mentioned above) uses the legend for her own twisted enjoyment while you are in Tranquility Lane. You can find what is supposed to be the Pint-Sized Slasher's mask in the Point Lookout expansion (In two locations, no less!)
Flandre Scarlet from Touhou. She is really 495+ years old but has the mentality and emotional maturity of a child. One of her main gripes is that whoever she tries to play with, except for other uberpowered Big Bads and main characters, tends to end up a splatter in the wall.
Rumia, being a man-eatingDarknessYoukai is this, too... Or, well, she would be if it weren't for the fact that she's far too moronically inept to actually catch a human. (Word of God has it she regularily blinds herself with her darkness powers and then flies headfirst straight into trees.)
Thief: Deadly Shadows arguably fits the trope with Gamall although since she's actually using the form of a girl she killed many years previously, Lauryl it may be an "in name only" example.
In the Total War series, one of the entourage members an assassin can get is an apprentice. The flavour text describes this trope.
Wendy in Rule of Rose isn't pure evil, but she is the sweetest of the orphans capable of charming a delirious serial killer into dog-like obedience. She is also completely in love with the protagonist Jennifer, and the feeling is mutual, at least up to a point. Unfortunately "sharing" is a concept she has no use for.
In Dead Space, we have Lurkers, though they actually qualify more as Fetus Terrible. In Dead Space 2, we have those, plus Crawlers (instead of fetuses, these are babies old enough to crawl...and they have an exploding sac on them) and The Pack (Bratty zombie kids...they don't have body-part weapons like most other necromorphs do, but they are fast and alwaysattack in groups)
Mad Father: the eponymous character's psychopathy began with killing small animals as a child. Then it got worse.It is also implied (and then revealed on a second run) that Aya was this throughout the game. Notably, this is also the reason why her aforementioned father wanted to make her into a doll; he couldn't stand the thought of his daughter being 'dirtied' like himself.
World of Warcraft has Wrathion. He's manipulative, cruel, genius...and he's only two years old. Yes, he is a dragon, but Blizzard even said that other baby dragons are not like Wrathion. Wrathion is the way he is due to all the experiments performed on him when he was still in the egg. In Pandaria, Wrathion is portrayed as the Only Sane Man alongside the bickering Horde/Alliance/Shado-Pan factions.
At least until the 'Siege of Orgrimmar' raid, where, after the player defeats Garrosh Hellscream, putting Troll leader Vol'jin in charge as the new Warchief, Wrathion reveals that he was rooting for the Alliance to win, having put effort into making sure they had their chance to dismantle the already-damaged Horde, becoming very angry that it didn't happen the way he wanted, and swearing that something bad was coming and he only wanted to prevent it.
Danny and Demi from very obscure Namco arcade game The Outfoxies. Conjoined twins separated by a train accident (never mind that conjoined twins are always the same gender), they took to contract killing. In their ending, they use the money they got from their assignments to buy a lot of cake, purchase, shave, and tattoo a small dog, and set fire to Disneyland.
Dahlia Hawthorne from Ace Attorney: Trials and Tribulations was a prime example of this trope before she had even committed her first murder. When she was just fourteen, she staged a fake kidnapping with her boyfriend Terry and stepsister Valerie in order to get revenge on her father by forcing him to give up a precious jewel, and then got Terry put on death row by faking her own death, with him as the only suspect. Of course, Valerie wasn't much better, but Dahlia was the one who came up with the plan in the first place, and again, she was only fourteen. And she later killed her stepsister.
Noblesse Has Kalvin & Regis. Kalvin is an Enhanced Human assasin who specializes in poison. Regis, while 199 years old, is still a child in both appearence & by he standards of his species, & while a good guy, he spends a considerable amount of time trying to kill people.
In The Gamers Alliance, both Arawn and Yurius were truly horrible even when they were still young. Yurius gleefully set people on fire, and Arawn massacred his foster family as well as plenty of other people who annoyed him but still had the nerve to act innocent in order to fool the mages investigating the murders.
Survival of the Fittest's answer to this trope is Kimmy Redmond, a girl who has the look of somebody five years her junior, complete with carrying a lollipop around. A unicorn lollipop. She also has very few compunctions about causing harm to her classmates. Although it hasn't happened yet, it's very likely this will transfer to killing them too. ( It didn't, but she tried.)
A more accurate example of this trope is 12-year-old prodigy Brandon Cuthbert, who, before the game, sliced up various woodland creatures for experiments. This carries over to when he dissects the former featherweight boxing champion of the world with a box cutter after suffocating him to unconsciousness with an X-box controller. He seems to show some remorse for his actions right before his death at the hands of Sera Wingfield though.
As part of the Doctor Horrible's Sing-Along Blog DVD release, "L'enfant Terrible" was one of the chosen winners for the Evil League of Evil Application contest. The application video can be seen here.
She's smarter than you.
Open Blue features Remillia, which deploys tykebomb special forces known as Angels. They begin active duty at the age of 13, and the younger ones tend to use their age to fool targets of their assassinations. Senseless Violins are just one of their many weapons.
By all appearances, Ness and Lucaslook cute. Too bad that in There Will Be Brawl, they're murderers who tore the organs out of two mob bosses, a policeman, a Pokémon, and Princess Peach. Even Ganondorf was scared of them.
The Mwa Mwa Penguins you can find at the Pet Shop in Club Penguin embody this trope.
Within the world of Magical Girl Hunters, some of the magical girls joined to save the world. Some just like the excuse to kill stuff. Captain Kawaii... * shudder*
The webcomic / video series Contemplating Reiko stars the daughter of Satan and her three older sisters.
Apparently normal to some extent for Homestuck's trolls. Backstabbing and mutilating each other is rife among troll kids, and Eridan and Gamzee went on killing sprees at the equivalent age of thirteen.
One thing a lot of readers forget until the killing sprees begin is that Vriska and Eridan had already murdered countless other troll kids and their guardians, prior to the game. A Million Is a Statistic, except when it's not.
As soon as Calliope and Caliborn's personalities split, Caliborn became an insufferable little brat whose first act was to bite into his adoptive father Gamzee's shoulder. He only gets worse with age.
Toki-Doki Rose (or Rose, for short) What'd she do back in Tokyo that would've landed her imprisonment had her mother not have left within 24-hours of them finding out? Well, she piloted a mecha-bot, destroyed half of Tokyo, and injured 200 people, making her far worst than her mother before her, something that got her compared to Stewie Griffin. Also, like Stewie, she is completely aware of what she is doing and she is about one. In fact, the vignette involving her secret is named "Infant Terrible".
Bonesaw, from Worm. A pre-teen Mad Doctor with the personality of a five-year-old. She has one Hell of a Freudian Excuse, though; she was forced to heal her dying family. Over and over and over again, until she was too tired to continue. After such, Jack, the man who killed her family, took her in, and Bonesaw followed the last request of her mother to 'be good' by putting on the superficial mask of a cheerful, well-behaved child while using her powers the way Jack wanted in order to stay alive.
On Roger Ebert's website he once posted a letter from a female reader who noted that it would be possible to save any number of mediocre movies by adding a scene at the very end which showed an otherwise normal looking child whose eyes suddenly glow. The example she used was the movie Volcano.
In Ultra Fast Pony, Apple Bloom is the preteen child of a mob family, and she's already taken to the lifestyle: blackmailing adults to get her way and burning down schoolhouses. Her friend Sweetie Belle is a creature of id, who destroys stuff on random whims and expects she can charm her way out of the consequences.
Dexter's dad from Dexter's Laboratory, apparently, when he and Dexter's mom turned into toddlers. During that time, Dexter's dad took pleasure at beating up Dexter's mom as a baby. Using Dexter's inventions to torture her.
Professor Princess from Transformers Animated dresses and acts like a young girl but has a psychotic obsession with destroying 'violent' toys, in one instance deactivating Angry Archer's explosive arrows because they were too destructive. However, she uses a wand-like item that destroys things in a blast of sparkles and flowers. Apparently if there's flowers it's okay.
Stewie Griffin from Family Guy is a comedy example; he's a sarcastic, ranting evil genius with a homicidal grudge against his mother, and he's only one year old.
Maybe Eliza Pinchley. Vowed to viciously kill Lois and meant every single word... She hasn't actually attempted it, though, and her character will probably never return to Family Guy.
And Penelope (voiced by Cate Blanchett), who is much like Stewie in the early days, except even more blood thirsty and sociopathic. She even managed to do something Stewie was unsuccessful at—killing her own mother.
In Batman: The Animated Series, Baby Doll is a washed up actress with a bizarre debilitating condition that makes her look permanently like a child regardless of her age. The fact that no one takes her seriously as an adult ruins her life and she becomes insanely fixated on the mannerisms and environment of her one successful role. Therapy can help, and she's actually pretty nice when she's not crazy. She just has a BIG Berserk Button.
José from Cybersix is the clone and "son" of the show's Big Bad, and carries out his father's plans in almost every episode. He's also a stuck up, ill-tempered, and bossy little boy.
Many of the younger South Park characters have moments of this:
Cartman, the racist, bigoted, murderous, self-centered Villain Protagonist. While his attempts at being cute aren't always very successful, he is still without a doubt, the most manipulative character in the show and is quite psychopathic.
Wendy Testaburger. In the episode "Tom's Rhinoplasty", where she gets hot substitute teacher Ms. Ellen shot into the sun by Iraqis in a fit of jealousy. In the episode "Breast Cancer Show Ever", she beat Cartman in a fight, and hit him so hard he started bleeding.
Professor Chaos! Bringer of destruction and doom!
Butters: "Society cast me out, and so I vowed to make them all pay! And pay they did! Nobody knows that beneath this sweet eight-year-old little boy lies the most evil, the most destructive supervillain of all time!"
Possibly Ike, who helped pull off a jewel hiest by pretending to be an innocent boy with an injury.
Bart's evil conjoined twin Hugo in a Treehouse of Horror episode: "A routine soul smear revealed the presence of pure evil." Though actually it turned out Bart was the evil twin and the results had been mixed up.
And considering that even in the present day she's about 12 and is perfectly willing to murder her brother and everyone that stands in her way of her father's throne... There's a reason her mother called her a monster when she was little.
Ben 10 had Ben's Evil Counterpart, 11-year-old sociopath Kevin 11, who was willing to kill hundreds of innocent civilians as part of a heist attempt in his first appearance (all previous villains on the show had fairly standard Saturday Morning Supervillain plots, Kevin's was the first to really be over the line by Ben's standards) and just got crazier from there. Unlike most examples of this trope, Kevin actually grew out of it in the sequel series. Kevin's also a Human/Osmosian hybrid, and the Osmosian tendency to go bonkers when absorbing energy doesn't help his sanity.
Suzy Johnson, Jeremy's little sister on Phineas and Ferb, acts sweet and innocent in the eyes of her brother, but when she crosses paths with his girlfriend, Candace, she shows her True Colors when Jeremy isn't looking.
She is also apparently the most horrible thing in the world to Buford, the local bully, who all the other kids fear.
Charles aka Brainchild from The Tick, a ludicrously intelligent kid (he mastered quantum physics before he could walk, among other things) whose desire to become a Card-Carrying Villain is met with benign acceptance by his hippie family.
Ren Hoek of The Ren & Stimpy Show was portrayed as one of these in the Adult Party Cartoon episode "Ren Seeks Help". As a child, he was playing innocent to his parents, but without supervision he is shown commiting acts of vandalism such as setting fire to buildings and torturing and killing animals. He gets away with it, until one of his victims (a frog) tells his parents about what he's been doing. After some lecturing on the horrible things he's done, they tell him the only thing he can do for the frog is put it out of its misery - rather than do this, he chooses to let the frog go and suffer more, leading to its eventual suicide.
The Boondocks episode "Smokin' with Cigarettes" featured Lamilton Taeshawn, a psychotic young boy from a broken family who lived with his grandmother. He claimed he did terrible things, not for sympathetic reasons, but because he liked to cause people to crash in their cars, break people's arms, and assault elderly people including his grandmother. He killed a dog and later attempted to kill Riley when Riley told him he didn't want to friends anymore.
The character of Lamilton was based on a boy named Latarian Milton, who stole his grandmother's car at the tender age of seven. When asked why, he responded "I want to do it because it's fun. It's fun to do bad things. I wanted to do hoodrat stuff with my friends," one of whom "smokes with cigarettes". The next month, young Latarian was back in the news — for beating up the same grandmother in Wal-Mart because she wouldn't buy him chicken wings. He was subsequently 5150ed.
Sarah from Ed, Edd n Eddy, Ed's little sister. She enjoys beating up the whole main trio, especially Eddy, who are all older than her and boys, for goodness' sake. Normally, she wears a facade that makes her looks sweet and nice, but she very often uses the argument of "telling mom" (and others, which include her attempting to suffocate herself) to manipulate Ed.
Porky Pig's toddler charge in the WWII-era cartoon Brother Brat. The kid's mom (a welder at Lockheed who leaves the kid for Porky to babysit) gives Porky a manual on child care. At the end of the cartoon mom shows Porky how to use the manual correctly (it's spanking).
In Adventure Time, Goliad becomes this after spending time at a daycare and seeing displays of authority and power succeed where diplomacy fails. This is especially bad since she's an immortal with Psychic Powers up the wazoo.
From My Little Pony: Friendship Is Magic, there's Diamond Tiara, who regularly bullies Apple Bloom for not having her cutie mark and in a later episode forces the Cutie Mark Crusaders to continue their tabloid racket with the threat of leaking out embarrassing photos of them.
And in Flight to the Finish, her mocking of Scootaloo's inability to fly more than a second or two nearly drives the poor filly to suicide.
6teen has Stanley, a young boy who attacks people with a ball launcher, embarrasses and insults anyone he doesn’t like, and destroys mall property. The worst part? His mom knows what he’s doing, and lets him get away with it.
Flick Duck of PB&J Otter is a mild example. He generally behaves very sweetly and nicely around adults, with "Yes, sir" and "No, ma'am" and all of that, but often gets up to mischief the second he's out of their sight, or at least he thinks he is. He's not without a conscience, though, and can also sometimes be genuinely friendly and nice.
Remy of The Fairly Oddparents who mostly wanted to make Timmy miserable because he was not as happy as Timmy.
Vendetta from Making Fiends. She lives up to her name, overuses the word "stupid", can make horrifying monsters out of everyday objects, shrunk her own parents down to rodent-size while growing her hamster to human-size, enjoys nothing more than to see others suffer, rules Clamburg with an iron fist, and couldn't be more than ten.
Barbie And The Secret Door has Malucia, who plans to take over the world at age 10 and uses a magic-draining scepter to scary effect.