Arlo the Alligator Boy: Arlo Beauregard's birth father, Ansel, grew up in the Brooklyn neighborhood of Seaside by the Seashore, where he was repeatedly teased and bullied by the other kids because of his half-bird appearance. It hit him so hard that he swore right then that he would hide his true self, posing as a regular person, and dedicate his life to becoming a successful self-made businessman, so he would have enough power to tear it all down. Upon seeing the newborn Arlo who was born half-alligator, he feared this would expose who he really is, leading to him making the biggest mistake of his life by disowning him and abandoning him into the sewers, kicking off the plot of the film.
Batman: Under the Red Hood: As with its source material, Jason Todd was a street orphan who Batman caught stealing tires of the Batmobile, leading the Dark Knight to adopt him as his ward and make him the next Robin. However, his cheerful and eager personality gave way to a more cynical outlook on crime, believing criminals didn't deserve a second chance. He was then pushed over the edge thanks to his murder at the Joker's hands and his resurrection by Ra's Al Ghul, at which point he begins targeting the criminal underworld upon his return to Gotham, lacking any sort of restraint that his mentor had taught him.
In Cars, Doc Hudson used to be a race car until 1954 when he crashed violently during a race. After getting repaired, he was replaced with the next rookie in line and never raced again.
Also Cruz Ramirez from Cars 3. She wanted desperately to be a racer just like Lightning McQueen and took up classes with much bigger cars, but by the time of her first race when the other cars were starting, she simply left the race behind and lost her only chance.
Marlin in Finding Nemo: his wife Coral and all of his eggs, except for Nemo, were eaten by a barracuda, which is why he's so overprotective of him.
Queen Elsa goes through her greatest failureas a child. She strikes her sister Anna in the head with her icy powers, causing her to fall unconscious. The trolls heal Anna and remove her memory of the event and her sister's magic, but she still comes out with a white stripe in her hair, thanks to the ice magic. In case you thought that seeing your sister's white stripe of hair and being reminded of your largest mistake wasn't bad enough, the trolls also warn her about the potential of mobs attacking her if she doesn't learn control, which also makes her fear for her life. Then her father tries to protect her from such a fate by keeping the magic a secret (even from Anna) and isolating her, locking her up in the castle. This essentially drives the entire plot and is the reason why Elsa is so detached from her sister and avoidant in general.
Anna is a naturally extroverted person who is forced to be isolated from others for the majority of her childhood. To make matters worse, she doesn't know why it happened, and from her perspective, her older sister suddenly cut off their friendship and then ignored her for no apparent reason. Her parents are implied to focus on her sister, and then they die when she is fifteen, and again she has no emotional support. Her sister still shows no interest in talking to her or even attending their parents' funeral.note Unbeknownst to Anna, this is because of Elsa's Power Incontinence, but Elsa keeps this a secret, so Anna doesn't know why her sister seems so uninterested in her. It's obviously left some scars on her, and is a big reason why she agrees to a quick engagement, because she's starved for affection and isn't familiar with what a healthy loving relationship looks like.
Megara in Hercules: she sold her soul to Hades in order to save her boyfriend, and he dumped her shortly after that. Also, it's hinted that she probably had more problems to face in life besides that.
It turns out that Po of all people has one, though he doesn't learn about it in full until just before the climax. It involves the attempted massacre of his entire species by Lord Shen and his birth mother sacrificing herself to save him.
Tigress. She was left by her parents in an orphanage, called a monster by the people around her and was not shown any signs of parental love by her foster father, Shifu.
Shifu himself; abandoned by his lying and scheming father and left on the doorstep of the Jade Palace, he eventually became one of Oogway's greatest and most dedicated students. Forming the original Five with his fellow pupils, he was forced to leave it all behind when Fenghuang turned to the dark side and Oogway's hand was forced to bring her down. His adopted son, Tai Lung, also soon turned from a sweet and affectionate cub into a proud, arrogant and violent individual (in part due to Shifu's parenting - which he knows) and his son broke his leg trying to get the Dragon Scroll, forcing him to leave his son in Chorh-Gom Prison, stripping him of his joy for a long time.
Lord Shen definitely had one of these. Neglected by his parents due to his weak, sickly physique and albinism, he began displaying signs of mental illness and sadism when he discovered the destructive side of fireworks. When he heard of the prophecy of a "warrior of black and white" destined to bring his end, he became desperate to avoid this fate, leading to his genocide against the Pandas. He was then banished by his parents for his actions, which he saw as a final sign they never loved him and became obsessed with conquering China to make something of himself.
Kovu from The Lion King II: Simba's Pride was raised by his vengeful mother to become Scar's successor and trained to be a hateful war machine so he can kill Simba.
The Little Mermaid III: Ariel's Beginning reveals how King Triton's beloved wife Queen Athena was accidentally struck to her death by the ship's hull, which led to him banning music from Atlantica and the merpeople from going up to the surface in grief. After the incident, King Triton became over-protective of his daughters — especially Ariel — and displays his racism towards humans for his wife's death.
In Quest for Camelot, Garrett reveals to Kayley in the past, he was accidentally blinded by one of the horses that he was rescuing from the stable fire. Thankfully, Sir Lionel still believed in him and taught him to adapt. Unfortunately, his situation worsened when he heard of Sir Lionel's death, prompting Garrett to go into self-exile into the Forbidden Forest.
In The Snow Queen (2012), The Snow Queen's backstory shows her as a girl whose natural sorcerous powers were so strong that she was rejected and mocked by her people.
''ToyStory2: Jessie used to belong to an eight-year-old girl named Emily who as devoted to western culture. Soon, she lost the one person who cared about her to make up and more feminine activities and picked up claustrophobia.
In Trolls, the reason why Branch lost his colors, refuses to sing, is a bit of a Grumpy Bear, and is massively (but rightfully) paranoid of the Bergens attacking is because when he was a kid, his singing attracted a Bergen, and Branch's grandmother got eaten when she tried to save him.
Lampshaded in Wreck-It Ralph: Sergeant Calhoun "Was programmed with the most tragic back-story ever." Her lover was killed in front of her by Cy-Bugs on their wedding day, and she blames herself because she forgot to do a perimeter check for enemies. It's part of why she's usually so on edge about the Cy-Bugs and aggressive.
Assassin's Creed (2016): Callum Lynch's father killed his mother according to the Creed to preserve the secret of the Eden Apple of Granada as modern-day Templars led by Alan Rikkin were coming for them. Little Cal didn't know any of that so he grew up an orphan hating his father and drifted in and out of correctional facilities.
Duke worked as a prostitute after she came to LA while still human. After that she was Made a Slave by Vlad after she caught his eye through his mind control powers, and was forced to serve in his harem for years before finally being able to escape.
Laurel is indicated to have had a very difficult time in earlier years, apparently having attempted suicide multiple times. This appears to be a result of bullying over being transgender, gender dysphoria or both.
Compulsion: Saffron suffered from an abusive mom who had pushed her toward stardom no matter what, causing Saffron's anorexia over anxiety as she constantly chided her to keep her weight low. Worst of all, she even had Saffron be raped by a casting director for her to get a role. As a result, Saffron is not only still anorexic decades on, but also wishes to die.
Subverted in The Dark Knight, where the Joker likes telling stories about his Dark and Troubled Past to explain his scars...but every story is different. This is a tribute to Alan Moore's The Killing Joke, wherein The Joker has the following line; "Sometimes I remember it one way, sometimes another...if I'm going to have a past, I prefer it to be multiple choice!"
In Death Grip, Kenny and Mark lost their mother in a house fire. They both have unresolved guilt over it. The fire was started when Kenny was playing flammable material outside and the mother died because Mark locked the door behind him due to his Ambiguous Disorder, trapping his mother in the burning house.
The first five minutes of The Descent kill off the main character's husband and young daughter, leaving her traumatized and sometimes hallucinating. A big part of what fuels her trip into Ax-Crazy, particularly if the monsters don't really exist.
In Desktop Desperadoes While it may be possible to sympathise with the main antagonist Pointer 95's feelings of rejection in his field of employment, to orchestrate the attempted fratricide of the superior employee is without question morally reprehensible and instantly eliminates any initial sympathy the audience may have briefly felt for him.
Fast Color: Ruth ran away from home due to her uncontrolled abilities, was hooked on drugs and then got pregnant (without knowing who the father is). Then she came home with her daughter Lila, but left when her powers endangered the baby, becoming a wanderer for years before finally returning.
The main character of Hoosiers, Norman Dale, moves to the small town of Hickory, Indiana to take one last shot at redemption as a basketball coach after ruining his coaching career many years earlier when he lost his temper and punched one of his own players.
In Horse Girl, Sarah seems quiet and content at first, but she has a number of dark details in her past life, many of which are only alluded to. It's implied that her mother suffered from a mysterious ailment, and that there was some accident involving her horse Willow that led to Sarah no longer owning Willow. Eventually it's revealed that Sarah's mother committed suicide only a year ago.
In The Hunger Games Katniss' father died when she was 11 in an accident that she suspects the government to have orchestrated. Without the father's income, the family starved and Katniss nearly died, which is why she is so survival-focused.
Both Eddie and Sarah in The Hustler (1961) have this; she even lampshades it when they first meet.
Sarah: Eddie, look, I've got troubles... and I think maybe you've got troubles. Maybe it'd be better if we just leave each other alone.
John had a loved one that froze to death on the same mountain that Pam found him on.
Jack, of Jack Brooks: Monster Slayer, had his family killed in front of him by a monster when he was a child. He ran, the only thing he could do, but it's eaten at him ever since, fueling his Unstoppable Rage and inspiring him to step up to become a hero.
Johnny Guitar's main characters Vienna and Johnny "Guitar" Logan have dark pasts dealing with their former lives as a prostitue and a ruthless gunslinger.
O-Ren Ishii from Kill Bill lost both of her parents to vicious Yakuza gangsters at the age of seven and then getting her revenge just four years later.
Kin (2018): Milly's and Elijah's parents had abused them (though Elijah doesn't remember this, since he was just a baby). She ran away from her parents, he was adopted by another family.
Something that happened "in Chicago" to Lucien Carr in Kill Your Darlings is repeatedly alluded to, much to his displeasure.
In Kingsman: The Secret Service, Eggsy's father died in action, and his mother remarried a jerkassdomestic abuser who liked to beat the crap out of Eggsy and his mom. At some point, Eggsy resorted to drugs and petty crimes. By extension his half-sister Daisy has this too, despite not even being old enough to speak yet: her own father is the domestic abuser and her own mother nearly killed her during the Valentine's Day Massacre.
Kong: Skull Island: Kong is the Last of His Kind due to having lost all his kin to the Skullcrawler horde, making him the last one standing against the creatures; and he's still only an adolescent. If Riccio's Maybe Magic, Maybe Mundane visions in the sequel graphic novel Skull Island: The Birth of Kong are real, then Kong's parents were devoured by Skullcrawlers right in front of him when he was only seconds old, leaving him traumatized from birth and with a lifelong rage kindled in him.
In The Lazarus Effect Zoe survived a fire that killed many people in her apartment complex. She's had recurring nightmares of it for years. Turns out she set it.
Listen to Your Heart: Ariana's father died from meningitis, which she caught and became deaf as a result. The loss quite clearly weighs on her still. Danny's mother died of cancer, and he was there at her death, which clearly caused a severe trauma since he kills himself rather than have anyone else witness him dying later.
Lost Girls and Love Hotels: Margaret's father abandoned her, then her mother died from cancer, and her brother developed schizophrenia. That's probably the source of her depression and suicidal desires.
Chisolm was a USCT in the Civil War and became a bounty hunter after witnessing his mother and sisters being raped and killed by Borge's hired men.
Jack Horne had a wife and children but lost them in an unspecified event. It's hinted that their deaths were related to him hunting and killing Indians for a government bounty.
Vasquez is wanted for a Texan Ranger's death but no details are given except Vasquez suggesting he had it coming.
Goodnight is a former Confederate sharpshooter suffering from PTSD and survivor's guilt.
Billy is a Korean immigrant who was an indentured servant who killed his masters and ran away.
In Manchester by the Sea, the protagonist Lee was once a cheerful and outgoing, if somewhat irresponsible, father and husband. Then, while he was high and drunk one night, he accidentally started a fire that burned down his house and killed all of his children. After being questioned at the police station, he attempted suicide, forcing him to leave town and become a recluse. As a result, he's been reduced to a shell of a man and a jerkass woobie by the start of the film, unable to forgive himself.
The eponymous Max Payne has one, his family was killed in a mysterious robbery. Made extra saccharine thanks to the Flash Backs being in a warm ember tone compared to the rest of the film's drab winter blues and grays.
To be even more specific, in the game the movie is based upon, his family when her family got killed by V-head junkies as part of an attempt to silence Max's wife for knowing too much about Project Valhalla. Tragically, Max's wife didn't have a clue what the memo meant and wanted to talk to Max about it, but he didn't listen to her because he had to go to work for the day.
Marvel Cinematic Universe: Even in a universe where superheroes exist, the heroes and villains alike have led some pretty miserable lives.
Gamora was only a child when her home planet was attacked by Thanos the Mad Titan, who killed her parents and half of her people in front of her. She became the favorite adopted daughter of Thanos, but she was forced to fight her adopted sister Nebula and was an assassin for him on a number of dangerous missions, witnessing countless deaths and travesties. She soon came to regret her actions but was still labelled a monster by those who suffered at Thanos' hands.
Nebula was often the loser in her fights with Gamora, so Thanos had her torn apart and made into a cyborg. She constantly strove to make him proud, but ultimately knew she would never be Gamora in his eyes.
Pretty much ALL of the Guardians of the Galaxy have had it rough, though special mention must go to Rocket who was an experiment on an alien planet who was constantly torn apart and rebuilt over and over again and kept in miserable conditions for most of his existence. He spent most of his life alone until he broke out and met Groot.
Erik Stevens aka Killmonger, son of Prince N'Jobu, grew up in a rough neighborhood in Oakland when his father was killed by his uncle, King T'Chaka and left him to fend for himself. He eventually became a skilled Navy SEAL, but never forgot the oppression he suffered at the hands of racists and the people of Wakanda for abandoning him.
Shang-Chi was only young when his mother was murdered in front of him by a former enemy gang of his father's. Afterwards, he was subjected to Training from Hell by his father and the Ten Rings, turning him into an assassin who murdered his mother's killer. He soon came to hate himself for what he'd done, so he ran away and intended to live a simple life in America away from his father.
Wanda Maximoff probably has the worst treatment of this. She was only a child in war-torn Sokovia when her home was bombed, killing her parents and a UXB (a Stark Industries missile no less) landed in her living room, trapping her and her brother Pietro underneath it. They were then taken by Hydra, who performed unethical experiments on them and turned them into the brainwashed young adults we were introduced to. It only got worse from there.
Natasha Romanoff was raised by "parents" who were secretly part of the KGB. One day after a close call with the U.S. Army, General Dreykov had her father imprisoned and she and her sister Yelena were taken to the Red Room, where they were permanently steralized and brutally trained to be soulless killers.
The Winter Soldier aka Bucky Barnes, Steve Rogers' best friend, was thrown off a train to his death in WWII during a mission with Steve. He survived, but was taken in by HYDRA, who subjected him to copious torture, experimentation and brainwashing in order to create the perfect assassin. He then spent decades Brainwashed and Crazy, killing many people (some completely innocent) and forced to forget his life as a regular American.
Miss Meadows: It's revealed that Miss Meadows became a vigilante because her mother was killed in a drive-by shooting as they were attending a friend's wedding.
Miss Sloane: Esme survived a mass shooting in high school hiding in the janitor's closet. As a result, she's become a gun control supporter who works at the Brady Campaign, but doesn't reveal this since people say her advocacy has been emotionally motivated. Sloane later reveals this publicly without asking, driving Esme to tears and outraging her, though she goes with it anyway afterward becoming the face of their support for a bill that would mandate guns have universal background checks.
My Days of Mercy: Lucy reveals hers to Mercy. Her mother was murdered, and her father is now on death row for doing it (but she doesn't believe he's guilty). His date for execution is only months away.
The emotional turmoil brought on by the childhood abuse of Tim Robbins' character is the main crux of Mystic River.
In Sidney Lumet's The Pawnbroker (1964), the main character is traumatized by what happened to him and his family in a WWII Nazi concentration camp. He hasn't been able to work through his issues, which causes those unpleasant memories to regularly flood his mind.
Jack/Kyra during the Time Skip between Pitch Black and The Chronicles of Riddick. She ran away to search for Riddick and signed up with a bunch of mercs, only to be enslaved, with a heavy implication as to what kind. Made worse in that she was only twelve at the time! Riddick is understandably pissed to learn this, as well as furious at her for ignoring his single instruction when they parted ways;
Riddick: I told you to stay on New Mecca. Did you not listen!
In Port of Shadows, army deserter Jean admits to Nelly that he is a wanted man and is planning to flee France under a new identity to start a new life in Venezuela. We never learn precisely what crime he committed, but it's serious enough that he can't stay to face the consequences.
The first five minutes of the Prom Night (2008) remake feature the protagonist coming home to find her father and brother dead, and watch her mother get killed, all because the killer was looking for her. For her character, this doesn't really come into play much for the rest of the movie, even when the killer comes back.
The Terminal: Gupta explains to Victor why he is so obsessed with keeping his janitor job: he was a poor tobacco dealer in India who was shaken down by a corrupt police officer. Eventually, Gupta stabbed the bastard in the chest after several days of extortion, and fled India with his family to not be charged with assault. By working as a janitor, he can remain off of anybody's radar. Dixon threatens to have Gupta deported if Viktor doesn't leave.
Gabriel in Van Helsing also has Wolverine Amnesia, it's implied by Dracula they both share "a history" together, and even hints at Gabriel being an immortal "tripped" angel or the like, but nothing concrete ever comes of it. Sequel Hook?
The Voices: Jerry unsurprisingly had one. He had a mentally ill mother who had hallucinations, which Jerry shared. To stop this, his stepfather tried abusing it out of him. This of course didn't work, and only made him want an escape with them even more. His mother tried to kill herself rather than be committed to an institution, and Jerry ended it at her request. Because the police found him with a bloody knife in his hand, Jerry was locked up (his psychiatrist mentions that he's on parole later).
Given that the main characters in The Way Back are seven gulag escapees and an orphan, this is something of a given. None of the men will tell each other their stories, but they all tell the girl, who, in turn, passes them on to the others. When she asks why they don't talk to one another, she's told that, in the gulag, the less you say, the better off you are.
The Wolfman (2010): You've had quite a nice little messed up childhood, haven't you, Lawrence? What with witnessing your mother's death and all, and being sent to a mad-house where you underwent all sorts of nasty treatments and then shipped off to live with a distant relative in America.
Magneto. "Holocaust survivor" is about as dark and troubled as it gets. He eventually settles down with a wife and daughter, only for them to die as well. It's no surprise Erik isn't exactly a ray of sunshine these days.
Wolverine's past history of fighting in the American Civil War, World War I, World War II, the Vietnam War, and his past work with Team X. How much of it still happened after the Cosmic Retcon is unclear, but we do get to see him escape from the Weapon X project in a flurry of adamantium and rage, so it's clear that the Broad Strokes of most versions of his origin still stand, and it ain't pretty.
In The Wolverine, Yukios first premonition was her parent's death, and after she was forced to watch it come true, she became a street urchin scavenging for food until Ichiro Yashida found her and adopted her under his wing.