Long time on again/off again X-Men antagonist/ally Magneto arguably fits this trope, having suffered through the discrimination of World War II as a boy only to face it again as an adult for being a mutant, though he's better known for being a Well-Intentioned Extremist.
X-Men: Madelyne Pryor learned that she was a clone of Jean Grey and was abandoned by her husband, Scott Summers (who fell in love with her because of her obvious similarity to Jean, though neither knew she was a clone at the time, assuming it to just be mere coincidence that they looked exactly alike), when he reunited with Jean, after Jean returned from the dead. Then Maddie's infant son was kidnapped and she was left for dead. She has a daydream where she's ripped apart to build the woman her husband actually wants and then is offered revenge. Thinking that it's just a dream, she accepts, which, naturally, leaves her possessed by a demon and willing to sacrifice her baby (who she finally has the power to find) to allow the demons of Limbo to take over the Earth.
The Scarlet Witch undergoes a fluctuating life where the good (a family with The Avengers, marriage to her One True Wuv, having her longed-for kids) is outweighed by the bad (her father is a supervillain, her husband gets mindwiped and divorces her, her kids aren't real), along with a number of possessions, kidnappings, and multiple forced amnesia inflicted by her most trusted friends. Then she rewrites the universe. Then she does it AGAIN.
Green Lantern villain Atrocitus. His entire space sector was wiped out by the Manhunters, and the massacre is enough to drive his anger to the point where it gives birth to the Red Lanterns. Not even the other characters realize just how painful seeing the massacre was until they visit his dead home world. "The Manhunters came and we... We did nothing to deserve this. We... we did nothing."
"He loved something once... life."
He even shares a brief moment of empathy with Saint Walker. Saint Walker admits that he, too, was once filled with rage after losing his entire family to random accidents during a pilgrimage. Only a brief moment, since Atrocitus points out one crucial difference: Saint Walker had no one to blame for their deaths, while Atrocitus can and does blame the Manhunters' creators, the Guardians.
All of the Red Lanterns are like this to one extent or another, since Atrocitus specifically looks for rage "driven by loss" when recruiting. Earth's representative, Dex-Starr, for example, is a housecat who witnessed the brutal murder of his beloved owner, then was put in a sack and tossed off a bridge. One was a woman whose husband was killed by the Sinestro Corps, she was imprisoned on Ranx and was repeatedly raped by the various Corps members.
The one exception to this is the newest member of the Red Lanterns as of Brightest Day: Lobo.
The Rage Entity Butcher's chosen host before it was sealed by Atrocitus was a man whose daughter was recently murdered.
And before it was retconned that he was possessed all along, Hal Jordan became this while he was Parallax. During that time, the other heroes tried to show to him that they could understand his loss and his pain, and convince him to turn back from the path he had chosen as they stood against him to protect the universe.
"What if the Avengers had become the pawns of Korvac?" (What If? v1 #32) ends with a giant Korvac sitting on Earth in a state of bottomless despair, holding the Ultimate Nullifier. He thinks of everything that ever was, is, and will be, and presses the button.
The Joker in the Emperor Joker storyline. He decided that any universe where not only his tragic past (whatever that was) could happen, but a guy like the one he became could be left alive and allowed to run rampant...well, a universe like that didn't deserve to exist.
In the last story arc in Captain Atom, Nathaniel Adam creates his own universe where he gets to control everything and create an ideal life for himself, one in which he never became Captain Atom. Being human, he then governs that universe so badly that he loses control of it to his own dark side, who forces him to relive his childhood in which his father left his fall-down drunk of a mother who proceeded to kill herself and his older sister, who had raised him. Finally, Cap is forced to destroy his own universe to stop his evil side. An unusual example in several respects: first, Cap is a hero; second, he succeeds in destroying his universe; and third, it had, by that point, become so monstrously horrible that destroying it was the right thing to do.
He picks up more experience than he'd like later on: during the WildStorm crossover Armageddon, he gets trapped in the Wildstorm Universe in such a way that he cannot leave and will eventually blow up, killing everyone in the Wildstorm universe. He does everything he can to prevent this from happening, but while he does eventually get cured and sent home, Nicola blows up the universe instead, but also recreates it in its post Infinite Crisis incarnation.
Furthermore, after being badly injured and put in a coma following the Infinite Crisis events, Captain Atom, making the best out of the hard learned lessons of the past months, personally captures, enslaves, and trains (by fighting to death in his personal arena) several alternate versions of people he knew, for the expressed purpose of conquering and laying waste to the entire Multiverse, in revenge for his past Woobie life. Or not.
Caliginous in Hero Squared has decided that life is nothing but pain, misery, cruelty, and death, and should be ended in preferably the most all-encompassing fashion possible. Her arch-nemesis, Captain Valor, just sees her as an evil megalomaniac, but his alternative self, Milo, manages to recognize that, beneath it all, she's a broken, lonely, psychologically tormented, and suffering woman.
A Marvel What If? shows us an alternate universe where Tony Stark fires satellite lasers at the Hulk during World War Hulk, killing every superhero present except for the Hulk. The Skrulls see the Hulk as their prophet and launch their attack early, killing most of the other heroes and conquer most of the planet within two months. The Hulk is recruited into the resistance, makes great progress, and becomes a symbol of hope and inspiration. Then the Skrulls hit the heroes with a bioweapon, killing them all— once again, except the Hulk. The Hulk, now utterly bitter and filled with nothing but rage, summons the Silver Surfer to have him call Galactus to destroy the earth and the whole Skrull population with it. Once this is done, the Hulk becomes the new Herald of Galactus, the World Breaker, and goes on to destroy countless other worlds by feeding them to his new master.
Scarecrow, anyone? Victim of a Prank Date, bullying throughout his school years, absentee parents, and an abusive grandmother with trained crows to attack him for the slightest mistake, no wonder the poor kid became obsessed with fear.
From Bad to Worse in Blackest Night. Due to being exposed to too much of his own fear gas, he can't even feel fear. Or nearly any other emotion anymore, except when facing Batman. Yeah, it means the Black Lanterns don't consider him a priority target and he brought it upon himself, but it's still a raw deal.
Joey Finklebarr, in Freak Force, is a boy who wakes up with super-strength and invulnerability. Once picked upon, he proceeds to murder his schoolmates and his abusive father. He is all but unstoppable, but gets talked down by Savage Dragon on the brink of total nihilism.
Shrinking Violet, the insecure wallflower of the Legion of Super-Heroes during the period that she was under the influence of the Emerald Eye in the Postboot continuity.
The Boys has recently gone into this territory. Butcher's response after finally having revenge on the man/superhero who raped and indirectly killed his wife who did not tell him about the rape, leading him only to find out several months later when the fetus grew exponentially and lethally clawed its way out of her womb: kill every superhero in the world.
Homelander turns out to be one. He only became a monster because of a horrific gaslighting campaign courtesy of his own clone Black Noir that convinced him he was already a hopelessly insane baby-eating rapist-murderer. Seeing photos of himself committing heinous crimes that he couldn't remember made him snap. Butcher remarks that Homelander became a psychopath by accident.
X-Force once faced a villain team called the Inner Circle, two-thirds of which were this trope. For starters, they all have uncontrollable powers related to death, which is bound to cause some angst. Individually, Eli Bard was a loser through his whole life, then was vampirized by Selene and forced to be her slave for the next two thousand years; Wither lost many of his friends (including the girl he liked) to crazy religious fundamentalists and later his surviving friends abandoned him for being "dangerous" (oh, and later he's vampirized, too); Blink was abandoned and left for dead by the X-Men after saving all their lives (to be fair, they Never Found the Body); and Mortis had an abusive father while her half-sister, Dazzler, had a loving one and grew up to become a famous singer. Selene gave them refuge and taught them to do whatever they wanted with their powers... which resulted in lots of people dying. While by the time of the final battle they're all ranting-and-raving Omnicidal Maniacs, that, if anything, makes their situation even more tragic. The two exceptions in the group of six are Senyaka, who signed up to have better opportunities to kill people (fittingly, he's also the one with the best control over his powers and the only one who was a villain before being recruited by Selene), and Selene herself, who just wants to become a goddess.
Doctor Destiny becomes one in The Sandman. Sure, he kills innocents to get his Materioptikon back. Sure, he uses it to project his own twisted, psychotic delusions onto reality. But his ramblings drive home just how sad and sick he is underneath it all. His first appearance in the series reveals that he is incapable of sleeping, and he's become a paranoid, shivering wreck stuck in solitary confinement, he barely looks human, and he's not allowed any clothes because he keeps attempting to hang himself. Then once Morpheus takes the stone back from him, he immediately breaks down crying and says he's sorry, and Morpheus believes he was totally sincere.
In the Loose Canon tie-in to My Little Pony: Friendship Is Magic, My Little Pony: Fiendship Is Magic, Sombra is presented as this, in contrast to the Generic Doomsday Villain of the show. Sombra was originally just an amnesiac orphan unicorn in the Crystal Empire's orphanage, a shy and gentle colt with a speech impediment who was constantly passed over by potential parents and picked on mercilessly by the rest of the orphans. He had only one friend, a ditzy unicorn filly named Radiant Hope. His Start of Darkness came when, on the first visit to the yearly Crystal Faire, the Crystal Heart showed him a vision of his becoming a terrifying monster, in contrast to his friend's vision of becoming a princess. Every year after, on that day, he would be left bedridden with immense pain, which got increasingly worse until one day, when he started slowly disintegrating! When his friend saves his life with a unique healing spell, he becomes convinced that his own descent into evil will soon follow. He promptly runs away from the orphanage, hoping to prevent this from happening, and in the process he finally finds out who he is: a creation of an Eldritch Abomination called the Umbrum that has been imprisoned deep beneath the Crystal Empire, sent to sabotage the Crystal Heart and set it free. This pushes him over the edge, convincing him that You Can't Fight Fate, and he went back to the Empire to steal the Crystal Heart. It wasn't until he was confronted by Princess Amora, who revealed she had known who/what he was all along and had simply left him in the orphanage, hoping he would somehow "choose good" even when he had nothing and nopony except Radiant Hope, that the last vestiges of his old self snapped and he truly embraced darkness. In-Universe, even Twilight Sparkle expresses sympathy for Sombra after reading his backstory in his old diary, although Cadance pointedly reminds Twilight not to forget that a sympathetic backstory doesn't excuse the evil he did.
Nemesis the Warlock: Nemesis's son Thoth witnesses the death of his mother and is accidentally abandoned for years on Termight by his father, who believed they had both died. He then tries to use his warlock powers to destroy everyone.
A downplayed example is the DC super-villain Doomsday; created by a twisted alien scientist who wanted to create the Ultimate Lifeform, his methods for doing so were utterly barbaric. Doomsday was engineered in a lab to be equipped with genetic memory... and then dumped on prehistoric Krypton, a lethal Death World. Afterwards, the scientist retrieved the remnants and cloned a fresh incarnation of Doomsday from the corpse before dropping him back into the wilderness. Countless horrific, painful deaths later, by a process of Lamarckian evolution, Doomsday has developed into the ultimate survivor... but has also been driven completely insane, so consumed with traumatic memories of thousands of deaths that what's left is a creature locked in a fear-fueled Unstoppable Rage, blindly lashing out at everything that lives because it's convinced that it can only be safe when all other life is dead.
The Death Queen in Lady Death is a would-be conqueror determined to take over Hell. She is revealed to be the main protagonist' mother who was kidnapped by demons and subjected to rape and torture. Feeling resentful that she wasn't rescued, she emerged as a extremely powerful sorceress and determined to take revenge on those she felt had abandoned her.
Evil Ernie from Chaos Comics was emotionally and physically abused by both his parents. As if that wasn't enough, he had telepathic powers that allowed him to find out that his neighbors knew about his abuse for years, but didn't do anything out of fear of retaliation because his wealthy parents controlled the business that was the number one employer in town. It was this that finally pushed Ernie over the edge, turning him into a serial killer and later, thanks to the original not-so-heroic Lady Death, into the mass-murdering spearhead of a zombie apocalypse that would ultimately wipe out the United States and an apocalyptic global nuclear war.
During the Vietnam War, an American soldier had an affair with a Vietnamese woman, and abandoned her shortly after. Their daughter was raised alone by her mother, until at ten years old she was separated from her, kiddnapped, and sold into slavery. That girl would grow up to become Cheshire, and would end up repaying the world's cruelty to her a hundred times over, becoming a lethal assassin, genocidal maniac, and all around horrible person.