In their strain to try and be taken seriously, teenagers are susceptible, gullible, and downright dangerous to anyone who wants to manipulate them. These are the best targets for demons, vampires or The Virus. Sometimes they're just delinquents with no respect, but are only a hair's breadth of sanity from shooting up an entire classroom, especially if you call them Just a Kid.
Much of this trope is fueled by a distrust in teenagers by the adult population. Not to mention, many parents of young children dread the moment when their innocent little angels become sassy, hormonally-imbalanced drama-magnets. However, television producers are often worried about offending the very lucrative 15- to 27-year-old market, so we'll usually see that the heroes are also teens, or at least one very "good" prominent one.
Ironically, the much milder version of this trope occurs with shows aimed toward younger children, who find teenagers cryptic, pushy, and intimidating for other reasons.
Some cases of Royal Brats can be attributed to this trope. This trope may also be lampshaded further still by invocations of how the teenage monsters in question used to be sweet kids. If teen monsters run society, it's a Teenage Wasteland. See also Big Brother Bully, Kids Are Cruel, and Humans Are Bastards.
Why do they act this way? Some do because they don't know any better. Some others however do, and know they will get away with it because of their age. Some again do so For the Evulz. Whatever the reasoning, the results are basically the same.
Of course, this can be, on an individual basis, Truth in Television; there are monster teens in Real Life in the same fashion that we can find some mean kids and cruel adults. The mere fact of being a teenager doesn't make people automatically good or evil; it does, however, make any given person more susceptible to reckless or selfish behavior than they otherwise would be as an adult, for biological reasons. This makes it all the more commendable and impressive when a teen acts maturely, responsibly, or selflessly in situations where it would be difficult or unexpected for an average adult to act that way.
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Anime & Manga
Mohiro Kitoh's manga titles tend to take a very dark view of teens and tweens. The best example would be Naru Taru, in which strange creatures called "dragonets" partner up with certain children, most of whom have issues with the way the adult world works, conferring upon them considerable power. Unlike other Mon-themed manga where that power would be used to protect Earth from alien or supernatural threats, many of the children become the threat themselves, turning their dragonets on each other and the adult world with horrific results.
Mohiro Kitoh's manga titles tend to take a very dark view of everything.
AKIRA has some great examples of this trope (with Tetsuo's being the most dramatic and disturbing).
Akira manages to both play this trope straight and deconstruct it. Whilst Kaneda and his gang are portrayed as teenagers that have little regard to the law, they are also shown as having a sense of honor (though it is small), particularly with Kaneda's feelings toward the death of a friend (in both the anime and the manga). By the end of the series, they actually manage to revitalize Japanese nationalism.
The teenagers in Battle Royale are forced to become monsters and kill each other. Some take quite easily to this new role, others never do.
Death Note: Light Yagami. He starts committing genocide on the criminally convicted at the tender age of 17.
Mello, too. He runs away when he's almost 15 years old, and when we see him again at 19 he's running the mafia with an iron fist.
Great Teacher Onizuka: The lead character is homeroom teacher of a class full of bullies, blackmailers, manipulators and scheming sadistics that hate all adults and believe that it is their mission turn every teacher they are given crazy, depressed or suicidal. However every delinquent is given a Freudian Excuse and a Backstory explaining their behaviour and how they became "damaged goods".
While not necessarily teenagers, most antagonists in Yu-Gi-Oh! were in their early twenties or earlier when they started out, and were at that age once the feces hit the fan. There was Pegasus at age 22 when we first saw him, there was Malik/Marik who was a mere 16 when we saw him (and who had started down the path of evil before entering puberty), and Dartz who started at only age 21, and kept himself that way for who knows how long, and Yami no Bakura, who probably wasn't more than 20 in the original timeline. To say nothing of all the other minor antagonists over the course of the story that were never older than 25, teenagers must get a horrible reputation in their world.
Prior to Duel Monsters taking the priority of the series, Yugi went through a whole series of teenage monsters every chapter in the manga, Seto Kaiba being among them.
This didn't stop at the first series either. In GX, the majority of antagonists of the day were fellow teen schoolmates, the chief antagonists of season 2 were all early 20s or younger, and season 3's subplots and season 4's plot were all ultimately instigated by youngsters.
Avoided in the third series Yu-Gi-Oh! 5Ds, mostly. While Aki is only 16, and goes on to terrorize Neo Domino as the Black Rose Witch and gets called a monster until her Heel-Face Turn, that is, all of the Dark Signers are adults.
All the contestants in the tournament in The Law of Ueki are supposed to be junior high students. Yet, many of them do not mind using their powers to Take Over the World, and apparently don't have any qualms about killing their opponents.
Miki from Aishiteruze Baby gets bullied by her classmates after she walked in on her teacher and her teacher tells everyone else "Pick on Miki. Get grades". Granted, one could make the claim that it's the teacher who's monstrous, but the classmates still went along with it.
It was a little more complex than that. Miki went to an extremely competitive school and saw a teacher beating up a fellow student for getting a low grade; the teacher left off with "Tell everyone you fell." The teacher never notices her, but Miki talks with her peers about it, none of whom are willing to tattle. One day, though, some of her classmates pull out an exacto-knife, hold Miki down, and go all-out on her, saying that they wondered what it was like to hurt someone else for once. It starts with the teacher and how he treats them, but he has no part in what the students do — he neither goads them into hurting anyone or suggests they do, and has no knowledge it's going on; it's all their own decision.
Arisa (actually her twin sister Tsubasa in disguise) has been ostracized, drugged underwater, beaten, and pushed off a cliff, among other attacks by her classmates, because she's trying to find the "King" who has an almost hypnotic hold on her sister's class. Tsubasa/Arisa's friend who previously lost faith in the King was ostracized and Driven to Suicide and is now paralyzed (it appears to be psychosomatic), but after finding out about Tsubasa being Arisa, she beat Tsubasa's aunt unconscious.
Carris Nautilus of After War Gundam X is a 15-year old boy who is willing to use increasingly extreme measures to bring the world under control. He eventually wises up. Shagia and Olba Frost, on the other hand, a pair of 19-year old Ace Pilots do not, and their Wangst over how unfair life is treating them nearly triggers an utterly devastating war.
Shinn Asuka of Mobile Suit Gundam SEED Destiny is hardly evil, but as a young, impressionable, and emotionally damaged teenager he easily falls victim to Chairman Durandal's toying with his emotions, and is used as the Chairman's attack dog for much of the show.
School Days anime involves a number of teenagers doing false "dating practice" with sex, betraying, harassing, blackmailing, cheating, avoiding responsibilities, raping, and murdering.
Explored in Attack on Titan, thanks to the series' focus on Child Soldiers and how sometimes Humans Are Bastards. While most adults commit horrible acts fully aware of what they are doing, many of the 104th Trainee Corps express callous attitudes or commit cruel actions because their inexperience or emotional immaturity.
Eren is downright terrifying, thanks to his Unstoppable Rage and his past as an Enfant Terrible, often leading him to make decisions that has negative consequences on him and others.
Mikasa is obsessively attached to Eren and she has become suicidally reckless when he is in danger. She has flat out stated before that she will hurt and even kill anyone, whether Titan or human, that stops her from getting to him and only cares about Eren and to a lesser extent, Armin.
Jean is callous when his hometown of Trost was being attacked, complaining he was one day away from joining the Military Brigade and leaving the whole mess behind. His attitude changes after the battle.
Before enlisting, Sasha hated the refugees from Wall Maria for the famine her village suffered through and believed outsiders shouldn't be helped. Her father challenged her to enlist and change her selfish attitude.
Krista was willing to get herself and Daz killed, all for the sake of dying heroically. Ymir calls her out on this.
Reiner, Bertolt, and Annie might be incredibly tragic, but they're still responsible for the events that killed off 20% of the human population. While their motivations remain unknown, all three are incredibly ruthless when it comes to accomplishing their mission. Reiner claims they didn't know anything when they began their mission, and all three definitelyfeelremorse over their actions.....but their youth, relative inexperience as spies, and unstable emotions combined with their power makes them an absolute nightmare.
Ymir is incredibly callous and calculating, thinking only of herself and the girl she loves, Krista. She's unapologetic in weighing the lives of the people around her against her personal goals, and leaves everyone around her uncertain of her true motives or goals. She admits to being a horrible person, willing to risk Krista's future just to see her one last time and emotionally manipulates her. Unlike her peers, she's painfully aware of just how much of a screwed up person she is, and keenly aware of how messed up everyone else is as well.
In the graphic novel Black Hole, teenagers are being afflicted with a mysterious STD that causes them to mutate randomly, occasionally turning them into literal monsters: barely human creatures that live in the woods. At least some of them seem to have become significantly mentally deranged because of this, turning them into more traditional monster teens.
Superboy-Prime of The DCU. In reality, he's just a teenager that had a serious mental breakdown. With all the immaturity and angst that implies.
This was America's prevailing attitude towards Young Justice in The DCU, causing them to eventually become America's most wanted super-group, despite the fact that they were actually pretty heroic and set a good example for others like them.
The X-Men and other Marvelmutants. Most mutant powers seem to manifest around puberty, and turn their recipients into freaks or outcasts of some sort.
The current incarnation of the Hellfire club are all teens at just about 12.
Elizabeth of Gemini Storm was recently revealed to be one of these, spending her teen years in juvenile hall and mental hospitals until government funding ran out from the quarantine and the institutes closed down.
Terra I from the 1980's "Judas Contract" arc of The New Teen Titans is a perfect example; while she initially joins the team as a hero, it is eventually revealed she's a sociopathic, emotionally unstable mercenary who deliberately joined the team as The Mole on behalf of the vengeful assassin she's sleeping with. She gleefully turns on them at his say so and almost gets them all killed, only stopping when she believes he's been cheating on her and lashes out, accidentally killing herself in the process.
An unusually serious and dark CATverse story, The Pćan of the Bells, gives us a flashback to The Scarecrow's incredibly shitty childhood. A group of teenagers enjoyed hunting him down daily, beating the crap out of him, verbally abusing him, and more, purely For the Evulz. He was terrified that he might just be killed sooner or later, and the kicker? They weren't doing this to somebody their own age or anywhere near their own size; Jonathan was still a little boy. Not surprising that he grew up to be a villain.
The Megamind fanfic Heroes makes it clear just how cruel and uncaring some teenagers can be compared to others. In some places the cruelty is shockingly realistic.
Battle Royale takes this to an extreme. After children in Japan start refusing to go to school, the government starts kidnapping entire classes of teenagers, putting them on an island, give them guns and other weapons, and tells them to all go kill each other, last one alive gets out free. Many of the teens, instead of fighting against the government, see this as an opportunity to wreak havoc, and start mercilessly killing off their classmates.
The films of Larry Clark; Bully, Ken Park and Kids. Ken Park features (NSFW) a teen stabbing his grandparents and getting an erection from it.
Similarly, the film Kidulthood, being centered around the troubled youth of urban London, feeds off of this trope - even a fair few of the protagonists are pricks to some extent, while the main antagonist is a towering Jerk Jock who batters his girlfriend and shows no visible regret at leading a bullying campaign against a girl at school which ultimately drove her to suicide. Combine this with the film's other underlying theme of Adults Are Useless, and you get a pretty bleak movie.
Odd Girl Out: Another Lifetime Movie. In this one, the protagonist is bullied both inside and outside of school to the point where she cuts off her own hair (after the other teens say her hair is too frizzy) and overdoses on sleeping pills. Even after her attempted suicide, the kids would not ease up on her. They posted a video of her being wheeled into an ambulance on a hate page that was dedicated to her and called her a "pathetic druggie".
13: With all the drug usage, underage drinking, underage sex (and numerous other illegal acts) that goes on in this movie well... lets just say this film is a parent's worst nightmare on what will happen when their sweet little angels become teenagers.
The Last Exorcism's Caleb is rather creepy and antagonistic and is actually a member of a demon-worshipping cult. His younger sister Nell, who's possessed and going to give birth to the demon, might be in on it too.
The Class (2007) is a brutal and realistic look at the effects of teen bullying. And how a whole class of students can gang up on an individual.
Downplayed in Where the Wild Things Are: Claire's friends don't wreck Max's snow fort out of malice but out of playing too hard for the smaller kid. They even look genuinely upset when they realize what they've done — but are much too self-conscious to apologize.
Jodee Blanco's memoir of recovering from intense bullying, Please Stop Laughing at Me, turns both this and Kids Are CruelUp to Eleven. A group of kids Jodee was friends with in a new town fall out with her after she refused to play a prank on a disabled teacher. They then come back to her over the summer, invite her to play baseball, then knock her out with a fastball, moon her, and call her a variety of names. Not only that, her crush wrote "You're going to have to fuck yourself bitch" in her yearbook, gets beaten up by the entire football team, and is made fun of when word gets out that one breast is smaller than the other, and that's not even the half of it.
Speak has Melinda's classmates. Heather jumping ship on her. Those kids at the pep rally. And Andy Evans who raped her.
Figures rather prominently in many novels by Stephen King - one almost suspects King believes Teens Are Monsters himself. Among other examples, we have:
Todd Bowden from Apt Pupil, the second novella from Different Seasons. Todd learns that his elderly next-door neighbor is a Nazi fugitive, but doesn't turn him in because he wants to learn the "gooey stuff" about the Holocaust. As his Odd Friendship with the Nazi continues, Todd graduates from dreaming about raping concentration camp inmates to becoming a hobo-mauling serial killer. Finally, Todd kills his guidance counselor and snipes motorists on an expressway.
Junior Rennie and his posse in Under the Dome. Three of them rape a girl while the only female member of the posse cheers them on, and Junior repeatedly has sex with the bodies of two girls he murdered and stuffed in a pantry.
Joffrey Baratheon from A Song of Ice and Fire is only 13 years old, yet is regarded by many as the single most depraved character in the series. Gregor Clegane was barely in his teens when he burned his little brother Sandor's face after Sandor played with some of Gregor's discarded toys that Gregor didn't even want anymore. Gregor only got worse as an adult.
Sixteen year old Ramsay Snow also qualifies, given how his main hobbies are flaying and hunting women for sport, and to a lesser extent nineteen year old Theon Greyjoy also counts, although he does bad things less because he's evil and more because he's a stupid kid that gets in way over his head. Of course, that all changes when the two meet...
In John Saul's House of Reckoning, the main character is crippled from a car accident and the school (and the town) whispers behind her back and alienates her, then accuses her of worshiping the devil. Because she has a limp.
Saul actually has several novels with outcast teenagers being victimized by their monstrous peers. The Unwanted, Black Creek Crossing and Punish The Sinners are a few examples. Then there's Teri, the glamorous teenage serial killer in Second Child.
In Andrew Vachss's Burke books the protagonist often passes by disaffected teens who may be violent, though they're rarely the focus.
Brought up in Discworld, but in typical Pratchett fashion, quickly subverted soon after. The Ankh Morpork teen street gangs are mentioned to be ruthless and deadly... but find themselves helpless to resist Captain Carrot's good-natured effort to organize them all into a Cub Scout equivalent, camping trips and chanting all included.
Gone, by Michael Grant. Partially justified in that the series uses an Only Fatal to Adults situation and every main character is 14 or 15 years old. On the other hand, there are a lot more villains than heroes, and the "good" characters have issues.
In The Butterfly Revolution, John Mason rapes one of the female campers, Blackridge tortures Divordich, and Stanley kills Mr. Warren.
The young Tom Riddle is an exaggerated example, who murdered his father and grandparents, framing his uncle. Opened the Chamber of Secrets, killed a girl and then blamed it all on Hagrid.
For that matter, James Potter and Sirius Black were examples of this trope regarding Snape, at least during fifth year.
Snape might be an inversion because, while he didn't even seem to be mean, he had a very dark mind, with his fascination with the Dark Arts and the creation of some violent spells. This is likely what his classmates' perception of him was.
And Dudley before the Dementors.
This is, essentially, the point of Sergey Lukyanenko's Knights of Forty Islands, where a bunch of teenagers are kidnapped by aliens and put on small islands connected by narrow bridges, where they're given anger-activated swords and told that anyone who conquers all 40 islands gets to go home (The Cake Is a Lie, as all "kidnapped" kids are copies). The protagonists decide to make an alliance with the neighboring islands in order to peacefully or forcefully bring all islands under one rule. While it works for a short while, teenage hormones soon take over, and the alliance falls apart, as many of them just want to have power and sex (two of the betrayers are murdered in cold blood by the protagonist after they rape a girl).
Ashes by Ilsa J. Bick takes this to the extreme. In this Zombie Apocalypse, an electromagnetic pulse has made most teenagers become Technically Living Zombies and killed most adults, thus making the only survivors younger children or elderly people, who both have no problem mistrusting teenagers.
Some of the teens in The Geeks Shall Inherit the Earth are unnecessarily cruel. The leader of Danielle's hate club actually wishes that Danielle dies from the anesthesia when she goes to get dermatological surgery.
Lord of the Flies pretty much breathes this trope. William Golding did not have a very sanguine view of the ability of human beings to solve problems without resorting to violence, and showed it through the preteen and teenage characters in the book.
Jesse, Theresa Klusmeyer and Harmony got turned into vampires.
The Pack: Hyaena-possessed bullies
Lampshaded heavily here:
Giles: Xander's taken to teasing the less fortunate?
Giles: And, there's been a noticeable change in both clothing and demeanor?
Giles: And, well, otherwise all his spare time is spent lounging about with imbeciles.
Buffy: It's bad, isn't it.
Giles: It's devastating. He's turned into a sixteen-year-old boy. Of course, you'll have to kill him.
Marc the Demon
The Inca Mummy
The Swim Team
Vamp Willow and Vamp Xander
Faith, until her redemption
Connor in Angel is a perfect example. The bad guys hardly have to break a sweat to manipulate the kid into doing really nasty stuff. Fortunately, he returns in Season 5 as a much saner person, due to having memories of a normal loving family instead of a childhood in a hell dimension.
The Middle Man plays with this trope. In one episode, it appears that a teenage Stalker with a Crush girl is following her favourite boy band group, opening vortexes in physical space and is about to end the world. Turns out, the girl was a General from another planet, sent here to stop the boy band, who were actually alien terrorists in disguise.
Kevin the Teenager in the Harry Enfield and Chums sketches. ("That is SO UNFAIR! I HATE YOU!") However this is more along the realistic lines of making him an unbearable, moody, whining brat who hates everything his parents do or say and is entirely controlled by hormones and current trends.
In the earlier Harry Enfield's Television Programme, Kevin (also called "Little Brother") was a hyperactive and relentlessly cheerful preteen. The first episode of ...And Chums showed his thirteenth birthday triggering a hideous Transformation Sequence, similar to Doctor Jekyll, in front of his horrified parents.
The Late Show with David Letterman amplifies this trope to its comic extreme in its Dwight the Troubled Teen skits. At the slightest provocation, Dwight loudly proclaims his hatred for his family and storms off.
Any episode of Star Trek: The Original Series (TOS) that featured anyone younger than 20 years old adheres strongly to this trope. In no fewer than three episodes (Charlie X, Miri, And The Children Shall Lead) unsupervised youngsters nearly destroy the Enterprise and/or kill the principal characters.
Trelane, the teenage omnipotent being who'll hunt you down if you ruin his fun.
Though he was 'born' within the run of the show, Q's son on Voyager was depicted as a teen when he made an appearance. He was also quite fond of making it absolutely clear that A God Am I to anyone who crossed his path.
In contrast, Naomi Wildman follows the same path of being born during the show's run, but was depicted as a child when she interacted with the crew.
Averted with characters the writers actually liked. Wesley (TNG) and Icheb (VOY) were stock Teen Genius characters who were generally well-meaning and sociable. Although Wesley was widely seen as The Scrappy by a segment of the fandom (and the Enterprise crew half the time), this had more to do with him bordering on being a Creator's Pet (and, less overtly, an Author Avatar).
The first couple of seasons of Without a Trace seemed to be dedicated to the idea that teens were vicious amoral beasts who lived to victimize each other. Co-incidentally(?), CSI: Crime Scene Investigation went through a similar phase about that same time.
The Sontaran Strategem, an episode of Doctor Who, features a genius teenager as an antagonist whose motivations can be boiled down to being a teenager. However, he came around in the end, sort of.
Also, he had gathered a large number of other genius teenagers to him who, on hearing his real plan, were appalled and immediately abandoned him. It implied that his plans were less about his age and more that he was somewhat unstable anyway.
Several Life Time Movies portray any teenager who's not a Naďve Everygirl or a squeaky clean Mama's boy as evil, rotten assholes or bitches who try to corrupt the good kids.
Early on in Andromeda, the ship docked at a station run by a tribal society of teenagers (everyone died young due to radiation poisoning, so the oldest survivor was sixteen). When the teens gained access to star-destroying bombs, they immediately went out and blew up inhabited solar systems with them.
The Tribe cast at some point, but particularly the Locos, the Chosen, and the Technos.
The House episode "The Jerk"'s patient of the week was a teen Jerkass that bullied his mother, insulted everyone he met just for the lulz, and was just a general all around brat (after getting tired of all the tests, when asked for a urine sample, he pissed on the floor). his mother even used the "he used to such a sweet kid till he became a teen" line. House only had his Eureka Moment discovery of the kid's illness by realizing that the kid's utter jerkassedness was NOT a symptom, he was just a teen A-Hole.
He also said it was probably the mother's fault, which usually is in more real cases of bullying or teenager rebellion.
Though not all, a lot of the meteor freaks of the week, people who usually go crazy and start killing people on Smallville, are teenagers. Part of that is likely due to the high school/college setting of the first five seasons, but it holds true in later seasons as well.
Initially an Enfante Terrible, Lx-15/Alexander Luthor plunges headlong into this trope, after he ages to the point where he's in his mid-teens. He attempts to assassinate Martha Kent, Clark, and Earth-2 Lionel, burns down Luthor Mansion, and attacks Tess, his Parental Substitute. Thank god that memory loss, and a Heel-Face Turn set in.
The Law & Order franchise is fond of this one — if there's a teenager in the episode, this trope trumps even Narrowed It Down to the Guy I Recognize if you're trying to guess the perp at home. Especially if it's a teenage girl. Bonus evil points to a girl in SVU who raped her little sister, convinced her boyfriend to rape her too, gave her an STD, got her boyfriend to kill the sister and another boy to kill him.
Subverted by the Power Rangers, where every teenager the audience sees ends up being unbelievably wholesome and respectable. Even the bullies and teenage-jerks usually turn out to be decent people when push comes to shove, although once every few seasons, a teenage character plays the trope straight.
Tommy's brainwashing as the Green Ranger notwithstanding, the few time we see this trope played straight are when one or more of the main characters is cast as a villain or as a brainwashed/corrputed character for most of the storyline, usually ending with a Heel-Face Turn about halfway through the season. Astronema/Karone (in Space), Mara, Kapri, and The Thunder Rangers (Ninja Storm), and Trent (Dino Thunder) are notable examples of this.
Lampshaded in the pilot episode; the original team is assembled when Zordon looks for "immature, overbearing humans" to fight evil. This was rephrased as "teenagers with attitude" in the opening credit sequence.
To be fair, Power Rangers rarely shows any evil human, regardless of age.
Judge Judy is a staunch believer in this trope. To be fair, though, most of the teenagers that appear on the show don't really do much to dissuade her of this opinion.
Judge Hatchett, a courtroom show that ran from 2000 to 2008, is somewhat of an inversion. Judge Glenda Hatchett tried many cases involving out-of-control teens and tried to get them on the right road through unique interventions, with varying levels of success. Contrary to Judy, Hatchett was an advocate for kids, often saying, "If we want our children to do right, we have to do right by them."
SCTV did "Nice Kids From Hell", a parody of 1950s juvenile delinquent movies, where a group of clean-cut, polite teens terrorize a middle-aged couple with their pleasant, helpful behavior.
Subverted somewhat in Teachers, in which the titular teachers, most of whom loathe their jobs, believe this about the teenagers they teach - but their actual interactions witht hen show that the teachers are more at fault than the teens. However, background events in any given hallway or yard scene do tend to show students casually setting each other on fire or hanging each other out of windows as the teachers walk by obliviously...
Accepted as a truth universally acknowledged in Glee, where it ends up being true at one point or another not just of the general school population but of almost every single one of the teenage protagonists too. So far Mike Chang is about the only one who's escaped it. The trope is taken to its furthest extreme in 2X01, in which Rachel is so jealous of an exchange student's superior singing voice that she sends her to a crack house instead of an audition. It is, however, also deconstructed somewhat with the implication that what lets teens be monsters is adults not doing the job of teaching them better: WMHS has an anti-bullying policy that is not enforced, but Dalton Academy for Boys features an enforced anti-bullying policy and Teens are emphatically Not Monsters there.
The highschool students attending campus for a program to earn college credits in Community episode The Art of Discourse wont stop MERCILESSLY hounding the cast for going to a community college, when they'll be one day going to universities. Strangely Up to Eleven - many fans felt they were too cruel to be funny.
In Madea's Family Reunion, the teens and young adults at the titular reunion are called out on their bad behavior, which included gambling, dressing inappropriately, and gyrating on each other.
Like his book counterpart, Joffrey Baratheon of Game of Thrones is a standout example. From ordering the massacre of his father Robert's bastards (nearly all of whom were children), to forcing one prostitute to sexually torture another for his own pleasure, to his emotional and physical abuse of Sansa Stark throughout the second season.
A frequent scenario on What Would You Do?. Actors depict teens in various bullying configurations—boys on boys, girls on girls, the victim targeted because they're gay, albino, or even red-headed—in public places, and we see how the adults nearby react (one thing we learn: adults are more likely to intervene when the teens wear black or otherwise look sort of gangsta).
Another one had a big teenage boy not only mouth off to his mother at a drugstore but do so in a way that strongly suggested he had physically abused her.
On Grimm, teen werefolk tend to be this, even more than adults. Like the violinist were-rat, the sweet little were-badger (or -wolverine; some sort of mustelid), or the hybred were-tortoise/were-lion.
In the first season, Numerius seems like a nice enough kid who worships Spartacus the way one would a famous athlete. He later forces Spartacus to kill his own best friend as part of an entertainment.
In War of the Damned, Tiberius takes this Up to Eleven, making Gaius Julius Caesar look like a Nice Guy in comparison! In his case it is shown that is father is trying to raise him to be a ruthless asshole, and simply did too good of a job of it.
The episode "Child Predator" has Adam Kemper the so called victim of kidnapper Samuel Abbot. He actually abused his kidnapper and forced him to kidnap and kill children, then pretends to be a guilt ridden victim of Stockholm Syndrome to get an immunity deal for his crimes. Thankfully Sherlock pulls a Rules Lawyer and get him convicted.
Episode "One Way To Get Off" has Sean Figueroa, illegitimate son of Serial Killer Wade Crewes, who's father convinced him to commit murders to make himself look innocent. He killed two couples, and a bystander who happened to be present.
My Chemical Romance's The Black Parade features the song "Teenagers", which starts off about how adults view teenagers, and then how, with the way school life has changed in recent years, why it's understandable they think this way. The end message seems to be that it's a vicious cycle.
Becky by Be Your Own Pet. The protagonist of the song murders a girl in revenge for high-school bullying. Apparently slightly based off Jemina Pearl's high school experiences. Without the murder.
The police procedural action strip Dick Tracy has a few teenage villains.
Jimmy White is an angsty seventeen year old who robs banks as well as his father, attempts assassinations, and acts as a cohort to major 30s villain Ben Spaldoni, who in turn worked for the Big Bad of the decade, Big Boy Caprice. He later explodes in an oil tank trying to kill another person. For a young kid that's pretty twisted.
Flattop Jones, Jr. committed at least two murders and led Tracy on a manhunt while trying to kill the aforementioned protagonist as well before getting killed. Considering this kid is the son of Flattop Jones, a mass murderer, hitman, and child killer, this is not unexpected of him.
Jeremy Duncan of Zits is basically a collection of all negative teenager stereotypes into one package. No matter what his parents do, he never seems to treat them with any sort of respect.
In Knights of the Old Republic. The cadets at the Sith Academy are early 20's at best, and indulge in things like beating up passers-by, and playing cruel tricks on the "hopefuls," like telling them to stand until they drop of exhaustion or starvation. Carth's teenaged son is a fabulously short-tempered brat who will try to skewer his own dad on a lightsaber if you choose the wrong dialogue option. Subverted in that Kel Algwinn isn't a bad guy, just unaware of other options, and that Mission Vao is higher on the Karma Meter than the party's Jedi because of her optimistic, trusting nature.
Bully gives you the chance to play as a pubescent terror. Student looks at you the wrong way? You can shove him to the ground. Nerd bumps into you in the hallway? You can knee him in the balls. Preppy student mocks your clothing? You can shove him through a glass window, beat him with a cricket bat, and repeatedly stomp on his head while he lays moaning on the floor.
Downplayed at times when the main character can be played as a compassionate teen and actively protect vulnerable students. And in the actual story, he usually only messes with people who have done something to deserve it.
Mass Effect averts this completely when you rescue the remaining students at Grissom Academy. The biotic students, all late-teens or very young adults, act like a family, and are all training to fight the Reapers on the front lines with their gift-abilities, knowing quite well how dangerous it is to do so. In fact, if you order them to act in a support role in the war (forming barriers for troops on or near the front lines), they'll protest it, but follow along the order. Regardless, they sometimes have to act offensively in self-defense even in the support role, and save a ton of lives with their actions, not complaining at all during their service in a particularly hellish war. The savants and intellectual geniuses of the Academy also act fairly mature and contribute wholeheartedly to the Crucible Project.
Booker DeWitt of Bioshock Infinite was only sixteen years old when he was at the Battle of Wounded Knee, which nowadays is often referred to as a massacre. It's implied Booker himself might be the reason why. It's revealed that Slate's soldiers gave him the nom de guerre of "The White Injun" because he collected so many grisly trophies from the dead, while Comstock's Voxaphone recordings reveal that after he was (correctly) accused of having Indian blood, he decided to prove them wrong by burning tepees down with the inhabitants (women and children) still inside. Since this took place before the point of divergence that turned Booker into Comstock, the Booker we play as is guilty of this as well.
In Beyond: Two Souls, a group of teens lock Jodie in a closet because she got a girl a book for her birthday. If you pick revenge, it doesn't end well for them.
In the semi-canon manga and game "The King of Fighters: Kyo", it's all but stated that Leopold Goenitz was already aware of his role as member of the Orochi clan when in his teens, and that he still was a teenager when he killed Maki Kagura, then a 10-years-oldgirl.
Jeffrey Rowland in this strip seems to be subverting this, and then says in the alt text, "When I was a teenager I probably should've been put in prison."
In Survival of the Fittest, some of the high-schoolers thrown into the game prove to be capable of horrifying acts of brutality, though many of the more violent ones are already insane, and a few are thrown onto the island from insane asylums and the like specifically to spice up the game. Even with that in mind though, the kids almost seem worse than the terrorists who abduct them.
Whateley Academy has what might seem to be more than its fair share of students who fit this trope. Possibly justified, though, in that it's presented as the only school for teenage mutants in the US (if not much of the world, given the number of foreign students showing up) — there just aren't that many other places for the offspring of infamous supervillains, the badly traumatized, and the just plain assholes among them to go, so Whateley quite naturally becomes the place where one almost by default encounters all of them at once.
The Bullies that plague the main character of Worm are bad enough that they drive her to near suicidal recklessness and induced a complete mental breakdown that was traumatic enough to trigger her powers. A later point is made of how despite this, none of her classmates got involved other then to join in. At no point do they show even a hint of remorse or even a reason for victimising her constantly.
Any memory that The Nostalgia Critic seems to have of high school involves some sort of bullying. In his Doug's First Movie review, he loses his temper at teen girls thinking the monster dressed up as female is cute, when in reality they cause so many body issues in each other.
Codename: Kids Next Door follows the milder version of this trope. It couples Adults Are Useless with the dubious situation of teenagers being loyal minions, perhaps based on the logic that teenagers actively try to behave like adults. Although they also betray the adults as well, possibly in reference to the fact that teenagers are also more rebellious against adults than kids are (or just to show that they are indeed bastards).
Many characters in the show enter an "outgrowing" phase shortly before their 13th birthday and become the rebellious bastard stereotype we know and love when they finally hit the big 13. Though not all of them are affected by this. Some teens (and a few adults) are hired as black ops double agents for the KND.
And there's Laura, who, whenever her Berserk Button gets pressed (In more ways than one), transforms into a monstrous teenager.
Superman: The Animated Series had an episode where Granny Goodness created a crime syndicate out of wayward teens in Metropolis, using Apokoliptian technology, love, and twisted sadism.
Two episodes of The Powerpuff Girls featured the Smith family, one member of which was their angry teen son Bud. The first episode, Bud's entire appearance consisted of him yelling something typically teenagerish at his father, such as "I hate you!" and "No one understands me!" The second episode, which features the entire family as villains, provides every member of the family with a motivation for why they hate the Powerpuff Girls - and Bud's is "I hate everything!"
There's also the Gangreen Gang, who are teenage monsters.
Azula of Avatar: The Last Airbender is nearly the lone exception to an otherwise subverted rule. She's the one prominent teenager in the entire show that starts and remains evil from beginning to end. However, she ends up being exposed as a very tragic, messed-up person, so not many people in-show or out of it tend to see her as a "monster" anymore.
Probably the next most prominent example is Jet, who attempts to kick an old man in the head and flood a whole village of innocents in his intense rage towards the Fire Nation. He's the leader of a gang of teenagers willing to commit murder and other dark crimes for no other motivation than revenge, and gets called a monster in-universe by Katara. He's changed his approach by the next time we see him, insisting that nobody else will get hurt when he tries to expose two firebenders disguised as Earth Kingdom refugees, but is forced to learn that Redemption Equals Death when his plan backfires on him.
The vast majority of villains on Static Shock are superpowered teenagers.
Mac's 13-year-old older brother Terrence in Foster's Home for Imaginary Friends. In the pilot (along with Duchess), he and her plan to kill Bloo, and most of his life is devoted to making Mac's life a living Hell.
One episode of King of the Hill has a gang of teenage paintball delinquents making Hank and his buddies' life a living hell (to the point of doing drive-by shootings with the paintball rifles) just because he took a stand for Bobby earlier. The trope's even mentioned when they get Bill to pull a Wounded Gazelle Gambit so Hank and Dale could pick off two of them:
Hank: You were right, Bill. Teenagers are cruel. They'll pick on the slowest, heaviest... Well, the important thing is, you were right, Bill!
Most of the Sherman High students in Sym-Bionic Titan are pretty cruel to the show's main trio when they start school there. Some of them get better.
Mother Up: Neighborhood teen Joel (the only teenager seen in the first season) hates everyone, especially his stepfather Greg who he repeatedly tries to maim, listens to harsh music, vandalizes things and plays ultraviolent video games.