Bowler Hat Guy (aka Michael 'Goob' Yagoobian) in Meet the Robinsons. The best friend/roommate of the main character, he is a gloomy boy who has endured a lot of the shenanigans his friend caused, but after being blamed for losing a game (due to falling asleep since he's often kept up), he's consumed by bitterness and becomes a bitter adult who never got adopted and became a pawn in a bigger scheme.
At the climax of My Little Pony: Equestria Girls Friendship Games, the human Twilight Sparkle, having come from a Friendless Background and been ostracized by her classmates, gives in to peer pressure to "Unleash the Magic" she has gathered from the Humane Five with her amulet, which transforms her into "Midnight Sparkle", a raging she-demon even worse than Demon Sunset Shimmer, and proceeds to tear apart the dimensional fabric between the human world and Equestria, threatening the destruction of the former if not both.
In Rise of the Guardians, Pitch Black, AKA the Boogeyman (as especially revealed in the prequel novels), believes that the fear he embodies is necessary and that the approach the Guardians take is foolish. But, since he hasn't been believed in for centuries, thanks to them, he tried to destroy everything they represent, just showing that his bitterness has destroyed any decency he once had. And when he tries to reach out to fellow loner Jack Frost, he is brushed aside. The look on Pitch's face says it all.
In Toy Story 2, Stinky Pete, the prospector, spent his whole life on a dime-store shelf, watching every other toy be bought by kids. This agonizing experience caused him to become bitter and willing to manipulate or outright force his "friends" Woody, Jessie, and Bullseye into going along with what he wants, whether they like it or not. Ironically, when he winds up being taken home at last by a little girl who defaces all her toys by drawing on them with crayons, he considers this the worst fate imaginable. However, according to the producers, it was stated that he accepted that fate and thus, averts the worst fate imaginable.
Steven Universe: The Movie has Spinel, who has become extremely bitter and miserable over being left to collect dust by her beloved master Pink Diamond for 6000 years under the guise of playing a game. The only reason she found out she wasn't even alive anymore was because Steven made a galaxy-wide broadcast catching everybody up on what had happened to the Diamond Authority. You can imagine how hard she snapped once the realization sunk in.
Oswald Cobblepot in Batman Returns: disfigured since birth, his aristocratic parents attempted to drown him in the sewers. He was found by a traveling circus, and was raised in the freak show as "The Penguin". While the public views him with sympathy, he has become a warped sociopath, plotting to murder all the firstborn sons of Gotham City. When Batman foils him, he straps rockets to his hundreds (thousands?) of pet penguins, intending to use them in a suicide bombing to kill all of Gotham, which, as the only setting we see, is extremely omnicidal in context. And yet, you still can't help but pity him at his death.
Bite: Casey. As if getting infected with a mutagenic virus and turned into a bug wasn't bad enough, her best friend betrays her and her fiancé cheats on her with said friend. It's no surprise therefore that she eventually rips the former's throat out and stings the latter several times before finally dying.
In Chronicle, Andrew is bullied and made fun of constantly through the movie, in addition to a sick mother and an abusive father. After he gets his superpowers, he begins to snap, and starts to get involved in crime to save his mother's life, stealing money for her medicine and such. By the end of the movie, during the climax, he nearly destroys the city.
Fantastic Beasts and Where to Find Them introduces us to the concept of an Obscurus, which is a rare and powerful magical parasite that is the end result of a powerful magical child being abused and forced to repress their abilities until they cant be contained anymore. At which point the resulting Person of Mass Destruction goes on a rampage destroying everything in its path, scared and angry after a lifetime of pain. A specific example would be Credence Barebone: Hes abused and controlled by his Fantastic Racist of an adoptive mother (along with his adoptive sisters, though he gets the worst of it), which has left him shy, deeply insecure and emotionally stunted. He has no friends besides his youngest sister Modesty and is mocked by the Shaws (particularly Henry Shaw Jr, who has no qualms about calling him a freak). This leaves him open to manipulation by Percival Graves/Gellert Grindlewald, who baits him with affection and the promise of teaching him magic so that the older man can use him for his own selfish reasons. And once Grindle!Graves gets what he wants, he has no issue throwing the poor guy away or calling him worthless. This leads to Credence hitting the Rage Breaking Point and willingly unleashing his Obscurus on New York City, which is as tragic as it is terrifying.
Godzilla himself in the original film. He was a normal dinosaur mutated by the Castle Bravo test in 1954, which gave him powers. This clearly made him stronger and angrier, not to mention in constant pain from his radiation burns. While destructive, he nonetheless is depicted as a Tragic Monster who just wanted to be left alone. After he destroys most of Tokyo, he goes to the ocean to have peace, but the humans won't let him be, and he dies rather in a Tear Jerker manner.
And again in Shin Godzilla. For a giant mutant monstrosity, this incarnation of Godzilla possesses a similar amount of tragedy to the original, with a ton of keloid scars, a mouth range that should not be physically possible, and glowing wounds all demonstrating this is perhaps the most miserable Godzilla to exist. It would not be an entirely inaccurate statement to say that he's essentially a giant cancer.
Gremlins: One particular Mogwai, stated as Earl in official media, is hinted in the film and novel as being one of the few Mogwai blessed with a more docile personality similar to Gizmo, that is until he is taken to the high school to be experimented on and then happening on that sandwich, condemning him to become another psychotic Gremlin and (presumably) suffer the same ill fate as all his other brethren.
J.D. (Christian Slater) in Heathers. For starters, his father is a sociopathic bastard who doesn't care for him (when asked if he even likes his father, he responds that he "[hasn't] given the matter much thought"), and his mother killed herself in front of his eyes to get away from her husband. His entire life was spent moving around from town to town and school to school wherever his father's demolition job took him, where, it is implied, he saw the same scenario of clique groups bullying other students at every high school he's attended. He starts out by murdering Jerk Jocks and Alpha Bitches and making their deaths appear as suicides (also implied as something he's done before), but he ultimately resorts to trying to blow up the entire school. He explains his intentions are such because he believes that nobody loves him, and that "the only place different social types can genuinely get along with each other is in heaven", somehow seeing the school as a representation of society itself.
How the Grinch Stole Christmas!: As part of the film's Adaptation Expansion, the Grinch is given this treatment. He was adopted by two elderly Whos as a baby, and bullied for his appearance, particularly by Augustus May Who, the current Mayor of Whoville. During one Christmas season, he made a gift for his crush, Martha May Whovier, but after being made fun of for having a "beard" at his age, tried to shave his face, cutting himself in the process. When his classmates ridiculed him yet again for his shaving cuts the next morning, that was the last straw: he went berserk and went on a rampage, destroying the gift he made and throwing a Christmas tree across the classroom before fleeing to the top of Mount Crumpit, declaring his hatred of Christmas.
Jurassic World: Owen makes a case for the Indominus rex being this, pointing out that being raised in total isolation with no other creatures to bond with, and possibly not even knowing what she is, likely contributed to her insanity and seeing the world outside her paddock as one big hunting ground.
Don Gallico in The Mad Magician. A quiet and peaceful man, his driving ambition is to become a successful Stage Magician. Just as he is on the cusp of seeing his dream comes true (as in literally standing on stage about to perform his greatest trick for an audience on the edge of their seats), his Mean Boss shuts down the show with an injunction that essentially means Gallico can never perform agaib, steals his greatest illusion and sells it to Gallico's greatest rival. Then his Gold Digger wife leaves him for the boss. Little wonder he snaps and starts murdering everyone who has wronged him. However, by movie's end, he is trying to murder his friend Lt. Bruce, who is merely doing his job and trying to apprehend a murderer, and admits to Bruce that he can no longer stop killing.
Loki in Thor. He wanted to to show his father that his brother wasn't perfect, and then found out that he was a Frost Giant, after being raised by Asgardians to hate the Frost Giants as monsters. He's the natural son of his adopted father's greatest enemy. Convinced his family has never loved him and Odin only kept him alive for some ulterior purpose, he sets out to prove himself as an heir. He was passed over in favor of Thora lot during their childhood. He becomes a literal example in the film's climax when he tries to use the power of the Bifrost to destroy Jotunheim.
Erik "Killmonger" Stevens in Black Panther (2018), whose story began with him finding his father dead in their apartment. His father was an undercover Wakandan prince who'd grown disillusioned with his country's isolationism, and helped Klaue steal vibranium weapons with the goal of starting a revolution in America. Erik, aka Prince N'Jadaka, spent the rest of his life gaining the skills he needed to find and take over Wakanda, while only growing *more* furious with their refusal to help the rest of the world. During the movie's climax, when T'Challa tells him that the war he's trying to start could just easily destroy the whole world, he retorts that the world already took everything he ever loved from him.
Midsommar: Dani's trauma starts off with the discovery that her sister killed herself and their parents, and then only goes downhill from there. She is highly prone to panic attacks after this tragedy, including one she has during a bad shroom trip. Between witnessing a gruesome ritual suicide of two elderly members of the cult and discovering that her emotionally distant and obliviously abusive boyfriend is cheating on her, it's no wonder she's finally pushed over the edge and becomes the cult's May Queen.
The Big Bad dogs in The Pack (a 1977 Survival Horror) are former pets whose Jerkass ex-owners abandoned them in the wilderness, eventually driving them feral with hunger. Their Token Good Teammate newest member is especially huggable, remaining friendly and timid instead of becoming Ax-Crazy like them... and not only ending up their Sole Survivor but even getting adopted by the main protagonist's family — in a surprisingly heartwarming Earn Your Happy Ending.
Davy Jones from Pirates of the Caribbean doesn't seem to want to destroy everything - just everything that crosses his path. He's like this because his one true love, the goddess Calypso, betrayed him (presumably for another man, though it's never elaborated on) centuries ago. Jones' agony was unbearable, so he cut out his own heart to end it. When that failed, he adopted a different tacticfinding relief by sharing his pain with everyone he meets.
In a perfect example of Break the Cutie, Alessa Gillespie from Silent Hill was burned alive by the cult she was a member of, but survived by her own power and remained wrapped in bandages for 30 years, unable to move anything but her eyes (and her lips, but only enough to kind of smile, but not enough to form words). She does the only logical thing and splits her soul into 3 parts so she can kill everyone in the cult (excluding Rose, her good half, Sharon, and her mother).
Star Trek Into Darkness: John Harrison/Khan, while an Evil Overlord back in the day, just wants to save his crew from Admiral Marcus, who forced him into making weapons for the Federation at the risk of having them killed. Harrison even calls them his family and starts tearing up about how he couldn't save them.
The Suicide Squad: Though he's a giant alien starfish that kills people and turns them into People Puppets, Starro was quite content to drift in space. He would have never come to Earth on his own, and did not seem intent on conquering anything (his surname of the Conqueror is given to him by Dr. Gaius Grieves to mock his captivity), at least not before being experimented upon for 30 years.
Us: Red is the de facto leader of the Tethered who is in truth the real Adelaide Wilson. Having been kidnapped and switched out by her Tethered at the age of eight, Red is subjected to replicating her Tethered's actions including eating raw, bloody rabbit meat; marrying Gabe's Tethered against her consent; and performing a crude C-section on herself when pregnant with her children. Desiring vengeance on her Tethered, Red orchestrates the Untethering, convincing the Tethered to invade the surface to Kill and Replace their counterparts. When mortally wounded by her Tethered, Red whistles the "Itsy Bitsy Spider" to reassert herself as the original by reminding her Tethered that she was just a clone. (And it's strongly implied she told Jason the truth about Adelaide, shattering his trust in her forever.)
Magneto/Erik Lensherr. Each film of the new timeline keeps piling on the trauma that fuels his rage. In First Class he was a Holocaust survivor who had to watch his mother die in front of his eyes, and worse, it was because he couldn't consciously use his powers at the time. In Days of Future Past the Brotherhood were killed and experimented on by Trask Industries. In Apocalypse, after giving living in peace a chance, his wife and daughter are killed after his cover is blown as a mutant. All of this leads to him becoming an extremist willing to kill countless numbers of people. Highlighted in X-Men: First Class and arguably even more so in X-Men: Apocalypse, since he's quite literally out to destroy the world.
Russell Collins/Firefist in Deadpool 2. The tortures he went through at Essex made him angry and destructive.
The titular character of Stephen Sondheim's Sweeney Todd: The Demon Barber of Fleet Street, after being sent to a penal colony for fifteen years on a trumped-up charge, returned home to find that his wife had poisoned herself (though contrary to what Mrs. Lovett would have him believe, she's still alive) and his daughter was adopted by the man responsible for his suffering, and, failing in his big attempt at revenge, finally goes Ax-Crazy and becomes this at the end of Act One.
From 35MM: A Musical Exhibition, the titular Sara Berry from "The Ballad of Sara Berry" is described as a "popular bitch", who is gorgeous, rich and has a hot boyfriend. But when Julie Jenkins is rumoured to be a shoo-in to win Prom Queen because she got her leg amputated, Sara's father makes it clear to Sara that he and Sara's mother will only consider Sara of any worth if she wins. Sara's single-minded obsession with becoming Prom Queen drives away her all friends and her boyfriend announces he's taking Julie to the Prom instead and her father tells her "there's just no future for a princess at Prom." Sara promptly snaps due to the pressure and desperation to win her father's approval and goes on a murderous rampage, killing six other girls and crowning herself Prom Queen, before Julie calls the cops. Sara is arrested and sentenced to an insane asylum, living in a fantasy world where she's convinced she is the "Queen of high school land".