Ken started out as a lonely child who was always in the shadow of his older brother, however after his death a series of events lead to him becoming the evil Digimon Emperor.
Oikawa started out the same way, having Iori's father as his only friend. But after Iori's dad was killed in line of duty, Oikawa promised himself he'd make their dream visiting the Digital World one day real no matter what, and with the help of Myotismon, he becomes a ruthless Magnificent Bastard who almost brought both worlds to ruins in chasing his mad dream.
Kekkaishi: Kuroda was once an Unskilled, but Strong samurai with a streak for killing. He then makes a deal with the Ayakashi and becomes the series' most memorable villain Kaguro.
Claymore gives us Roxanne of love and hate. When she became a warrior she received the number 35 and was a low-ranking warrior. To survive in battle she started to copy the technique of a better-ranked warrior which she also worshipped. She then grew better than said warrior and upon getting a better rank, she abandonned the technique. Worse of all she managed to have her former idol killed in an awakened hunt. Then she turned her attention to another better warrior and did the exact same thing. Again and again. She finally reached number 2. She got close to the number 1 but finding out her special technique and deeming it disgusting she didn't try to copy it. However she contrived a way to have the best friend of the number 1 killed so that the number 1 comes especially to kill her. Of course she alerted the organization to gather every warrior. During the fight she managed to take the head of the number 1 and got the number 1 for herself. After the death of Roxanne she is known as a major pain in the ass for the organization and one of the 8 strongest number 1 that ever existed in history!
Goku from Dragon Ball was this to the Red Ribbon Army. They first dismissed him as Just a Kid, before hiring a top-notch assassin to kill him, to full blown nightmare when he came knocking at their door. He continued to maintain this impression to Frieza as well. Frieza didn't even know that Goku existed until he flew right in his face.
Like his father before him Gohan was dismissed as Just a Kid to the point that when Goku claimed that Gohan could defeat Cell everyone, including Gohan, was doubtful to say the least. Then Gohan went Super Saiyan 2 and put the fear of god in Cell.
Dragon Ball: Yo! Son Goku and His Friends Return!! deals with Abo and Kado, members of Freeza's Planet Trade Organization that were originally ranked below the Ginyu Force that both became as strong as Freeza himself in the fourteen years following his death. And they can perform a fusion technique to become even stronger!
In Dragon Ball Super, before having an encounter with Goku, Zamasu was just a Kai training to become a Supreme Kai under Gowasu's instructions but somewhat has a negative disposition towards mortals. But after his meeting with Goku thanks to a Stable Time Loop, Zamasu eventually began descending into darkness, becoming from a talented but not much stronger than Super Saiyan 2 Goku to an Omnicidal Maniac whose incarnations have performed mass genocide of mortals across multiple universes and can easily overwhelm Goku and the Z-Fighters thanks to one having Goku's body and the other gaining full-on immortality. Fusion Zamasu is especially strong enough that he is capable of going toe-to-toe with Vegito, despite the fusion in question previously stomping Super Buu from earlier.Even when his physical body is destroyed, his hatred and madness becomes so great that his spirit turns into an Eldritch Abomination that infects and becomes Future Trunk's timeline and multiverse, killing all the remaining mortals and only defeated when Zen'O himself is summoned and destroys the entirety of that multiverse.
During the Tournament of Power arc, Kale, became this. While Caulifla considers Kale to be special enough to be her personal protegee, most people barely acknowledge the seemingly weak gang lapdog unless her mentor's around. After getting tortured and ignored by her enthralled boss during the tournament, Kale finally cracked into her Super Saiyan Berserker and made a fearfully destructive presence that literally reshaped the arena. This is enough to attract even the Pride Troopers, causing Jiren to quickly forced her out of her rampage, while Kahseral's squad specifically called her out as a motive for payback against Goku, Caulifla, and Kale.
Zeta Gundam brings us Paptimus Scirocco, a minor official from Jupiter recruited by the Titans. Over the course of the series he slowly gathers followers and influence, culminating in him supplanting Jamitov and Bosque and seizing control of the Titans. And did we mention he slowly reveals himself to be an amazingly skilled pilot, genius mobile suit designer and extremely powerful Newtype?
Mobile Suit Gundam ZZ has Glemmy Toto. Originally introduced as a minor, bumbling Neo-Zeon officer, he slowly becomes more and more ruthless and competent until, in the endgame, he's sending out mind-controlled Cyber-Newtype clones in psychically-controlled superweapons, and slaughtering Haman Kahn's top lieutenants left and right. It helps that, somewhere in there, he was revealed to be either a clone or the Bastard Bastard of Gihren Zabi, the Big Bad of the original Gundam.
In Mobile Suit Gundam Unicorn, Buch Concern, the salvage firm Banagher Links works a part-time job at, is run by the business-oriented Ronah family. The only real notable thing about them at that point in the UC timeline is that they had taken (and bought) their last name from an impoverished line of European nobles. In less than 30 years, that same family would have enough manpower, resources and influence to forge their own realm almost overnight: Cosmo Babylonia from Mobile Suit Gundam F91.
InuYasha's Naraku. A bandit fails in his attempt to take over his band and ends up a crippled, burnt wreck of a man destined to live out the rest of his life in constant agony. But his Perverse Sexual Lust for Kikyo grows so strong he makes a contract with demons and is transformed into the Big Bad.
Death Note: Disaffected high school student Light Yagami and incompetent, unpopular corporate drone Kyosuke Higuchi each come across a simple notebook leading each to become notorious mass murderers and, in the case of one of them, GOD OF THE NEW WORLD.
Fullmetal Alchemist: Father, the Big Bad and leader of the Homunculi, began life as a small Xerxian alchemy experiment who could think and act like a human. Eventually, he tricked a slave (Van Hohenheim) into helping him absorb the souls of half the people of Xerxes, gaining humanoid form. Soon, he introduced Alchemy to the rest of the world and became the founder of the entire country of Armestris, kickstarting the manga's plot.
Luffy went from being a no-name rookie pirate (sure, his family is kind of important, but it wasn't common knowledge at the time) from the weakest sea in the world, to having a 500 Million beli bounty, declaring war on the World Government and slapping down their elite Warlords of the Sea left and right
Teach was just a Giant Mook in Whitebeard's crew. Even when we first meet him in the story, he's just a random guy who nearly gets in a fight with Luffy over pie. Then we find out he acquired a "special" Devil Fruit… suddenly he joins the Seven Warlords of the Sea, kills the strongest man in the world and steals his earthquake powers, and declares war on the entire planet. Fast forward two years, and he's one of the four most powerful pirates in existence.
Buggy the Clown is an interesting example. On his first appearance, he's the arc's Big Bad. Shortly afterwards, he comes back to take revenge on Luffy, and the only thing that keeps him from succeeding is being struck by lightning. He doesn't appear again until a few hundred chapters later, when the Sorting Algorithm of Evil has left him in the dust. But one thing leads to another, resulting in him amassing a powerful crew from Impel Downand becoming a Warlord after the time-skip. Yeah, he's a nightmare in the eyes of the general public, but the Straw Hats and everyone beyond the fourth wall knows that he's just a Fake Ultimate Villain. On the other hand, Word of God confirmed once that Buggy falls under the Brilliant, but Lazy category; it is fully possible that he Took a Level in Badass during the time skip to become worthy of his position…but that's yet to be seen.
Another example is Eustass Kid, who managed to get his 470,000,000 Beli bounty by sheer quantity of criminal activities, rather than being regarded as a significant threat to the World Government.
Hody Jones, captain of the New Fishman Pirates, is an interesting example in that he's basically Arlong without any of his redeeming qualities or even a tragic backstory to justify why he's such a racist dick. He simply went from being a normal fishman kid growing up in the racially-charged Fishman District to eventually becoming a fanatical anti-human, steroid-doped monster strong enough to nearly destroy all of Fishman Island.
Charlotte Linlin was just an innocent child who had the misfortune of being born abnormally large, with innate destructive power and an uncontrollable eating disorder, all of which eventually forced her parents to abandon her. She then fell into the care of a undercover child slaver, Mother Carmel, who pretended to care for her but also enabled her destructive habits as a selling point to a potential buyer. After Carmel "disappeared", she fell into another enabler, her future head chef Streusen, who encouraged those same habits for kicks. Linlin, devastated by Carmel's disappearance, sought to fulfill her caretaker's false ideals, and took her fake dream and made it her own: a world where all races could live together peacefully and see each other "eye-to-eye" — by any means necessary. Thus, the future pirate Emperor "Big Mom" was born, and she continues to terrorize the seas to this very day.
And then it turns out that since the Noah's souls are those of the original survivors of the biblical Flood (notice the name?), everyone on the planet shares their lineage, meaning that absolutely anyone could potentially turn out to be their host. Can you smell the Paranoia Fuel?
Jellal Fernandez in Fairy Tail started out as a slave working under the oppression of a Religion of Evil. Watching his best friend and love interest be brutally tortured and being even more brutally tortured himself, along with subtle brainwashing by a false god, turned him into an incredibly powerful mage whose powers come directly from celestial objects (such as meteors, among other things). He also managed to pull off a perfect Evil Plan, and it was only by devouring Etherion that Natsu was able to summon the power to defeat him. He is later healed by Wendy and becomes a good guy, although he is still one of the most powerful characters in the series.
The members of the Oracion Seis (sans the leader Brain) started out as child slaves to the same Religion of Evil as Jellal. After Jellal went Brainwashed and Crazy, the five were chosen by Jellal's Evil Mentor Brain for having the highest Magical potential of the slaves and trained by him. Fast-forward to the present day and these five are the spearheads to one of the three most powerful Dark Guilds in Ishgar and so dangerous that Fairy Tail had to enter an alliance with three other guilds to bring them down and stop them from activating a Doomsday Device. Later on, four of the five said members would escape from prison and, together with a Starter Villain from a minor dark guild and a doll brought to life, reformed the guild and took control of anotherDoomsday Device, with Fairy Tail having to join forces with a formerly Corrupt Church (corrupted through the Oracion Seis' own actions at that) to bring them down again.
Zeref, the person Jellal was supposedly working to resurrect, was a case of this as well. Originally an otherwise normal, if absolutely brilliant, student at the Mildian Magic Academy he researched all manner of taboo subjects in an effort to bring his brother Natsu back to life after his untimely demise. Fast forward 400 years, and he's the most feared and powerful wizard in history with numerous dark guilds being formed in dedication to things they believe he would have wanted.
Tongpu aka Mad Pierrot from Cowboy Bebop was originally an average man who had volunteered for an experiment to become the ultimate super human, the resulting experiments drove him completely insane and he became a bulletproof, psychotic Serial Killer with the mind of a child.
The D-Reaper from Digimon Tamers. It began as an extremely simple computer program, designed to delete any other program that grew beyond its purpose. Then the Digital World came into being, and the Reaper decided that not only did the Digimon qualify, so did we. It also had an increase in power over its lifespan, going from ridiculously simple computer program to nigh-unstoppable, city-devouring Eldritch Abomination.
Suitengu, the Big Bad of Speed Grapher started out as a nice Japanese teenager who loved his little sister. Then, Evil Debt Collectorsdrovehis parentsto suicide, and he and his sister were taken as slaves- she was sold into prostitution and he was made into a Child Soldier immediately after some rich guy paid to rape him. After surviving a bunch of battles, he was badly injured, and some scientists decide to do a bit of Playing with Syringes and make him into a Person of Mass Destruction (he also gets a Bloodbath Villain Origin when he slaughters the soldiers who tried to cover up the experiment by killing him). After returning to Japan, within a period of about a decade, he goes from "some guy" to knowing (and knowing the dirt on) the most powerful people in Japanese society.
The title character started out as a Boisterous Weakling who graduated dead-last in his year, but has grown into a super-powerful badass.
However, as he was also the Kyuubi jinchuuriki and the Fourth Hokage's son long before he was badass, you could argue he was never a normal or nobody to start with. He was, however, presented as one as he was unaware of both of these things at the start of the manga, but the adults around him all knew.
Kabuto Yakushi starts out as an experienced Genin who's actually a high-level spy for Orochimaru, but he eventually surpasses his already legendary leader in power and ends up co-leading an evil army of resurrected legendary ninja and Venus Flytrap monsters created from the DNA of one of the most powerful ninja ever.
Big Bad Tobi, who turns out to be none other than Obito Uchiha. That's right folks, the same kid that spent his childhood failing at everything, pining for Rin, feeling jealous of Kakashi, and pretty much being the Naruto of his day, is now the guy who indirectly killed Naruto's parents (one of which was his own sensei) by taking control of Kurama and unleashing him on Konoha, took part in the Uchiha massacre, corrupted Nagato and later Sasuke, co-led the aforementioned evil army against the five great powers of the Naruto universe and is now working to unleash the universe's resident Satan (albeit depowered somewhat), to say nothing of a whole laundry list of additional heinous acts he committed between his "death" and the current story. Basically he's Naruto on Evil.
Pain was once a normal kid, not even a shinobi as well as a Street Urchin orphan, who with Jiraiya's teachings, became the leader of the most powerful evil organization, killed the original leader of the Hidden Rain, along with his entire network of people who knew him and became a Physical God who destroyed the Leaf village effortlessly.
Gaara's somewhat of a lesser example. As a child, Gaara made many attempts to connect to the people of Sunagakure, all of whom feared him because of his connection with Shukaku. Then his uncle, the only person shown to have ever loved and cared for him at all attempted to murder him under the orders of Gaara's father, the Fourth Kazekage. All of that was enough to turn Gaara into a sociopath by the age of six, and a serial killer by the age of twelve. Naruto's intervention was what would finally set him straight and help reverse some of the damage.
Ringing Bell has Chirin, who starts out as a cute little lamb, grow up and become a ram who just looks demonic.
The seemingly cute and harmless Misaki actually became an Eldritch Abomination threat to the multiverse in Tenchi Muyo!. The author had foreshadowed this but everyone had assumed he was joking until it actually happened.
Hunter × Hunter has the Chimera Ants, initially a species of small insects that take on traits of the animals they eat; other than that, they're garden variety bugs. There even first mentioned by a character as a dining delicacy. That's until a rare six foot tall specimen starts eating literally eating everything from animals to Nen users, creating an army of animal hybrids capable of fighting and overpowering the elite officials of the Hunter Organization. And her heir offspring eventually gets so strong, that he's the closest thing to a Physical God before the strongest human, Nukes him to kill him off in order to prevent him from destroying humanity, even then the blast didn't kill him, only the radiation sickness that followed it..
The very first villain to appear in The Big O, and one of the few to make a repeat appearance without being an outright Big Bad, is Beck - a no-name low-level crook who wound up working as a go-between and muscle for a secret conspiracy between a Mad Scientist and a Corrupt Corporate Executive. That is, until he betrayed them both and stole the Humongous Mecha they'd been developing in secret, and then proceeded to stomp through downtown in order to rob the Mint like an avaricious Godzilla. Between all the Humongous Mecha and Kaiju that the title robot has fought throughout the series, Beck's record remains the best, returning to challenge Roger four times - arguably five if you count using his abilities to make the Big Bad and The Dragon's Bigs operational in time for the Grand Finale. And on top of that, he's responsible for releasing the most destructive weapon in the series when he tells Dorothy how to activate Big O's Final Stage. He's very hard to take seriously at ANY point, but really, you probably SHOULD...
And in the manga, he manages to get his hands on a "Gigadeus", completely trashing the Big O and nearly winning in one of the trippiest sequences in the series.
A perhaps more conventional example is Schwarzvald - once upon a time, he was just an Intrepid Reporter, driven to discover the truth about the City of Amnesia while working on his great manuscript in a tiny, run-down apartment. Then his search for the truth drove him to explore the vast network of underground tunnels beneath the city, and what he found made him Go Mad from the Revelation. Next thing you know, he's reappeared covered in bandages, trying to teach people the 'meaning of fear', and generally acting like a combination of The Joker, Scarecrow, and a mummy. With a Humongous Mecha. Which can fly.
Griffith of Berserk began as nothing more than a simple commoner who had big dreams of becoming king. Over the years, he put together a band of thieves, which later became a mercenary army, and then became the highest-ranking general in Midland's army. He then suffered a tragic fall from grace that left him disfigured and physically incapable of realizing his dream...until the Eclipse passed, where he became one of the five demon kings of the Godhand and one of the singlemost powerful individuals in the world.
Guts, too: he Used to Be a Sweet Kid, until his adoptive father sold him to a fellow merc as a sex slave then tried to kill him in a drunken rage. In the time since then, he went on to become one of the most dangerous mercenaries in the land, and after the Eclipse, developed a reputation as the Black Swordsman, the bane of apostles and the one foe the God Hand respects.
In Project ARMS, Alice was a sweet girl (albeit a genius, but no one outside of the Egrigori knew of her existence) who took care of the other experimental children and considered Keith White to be her dear "father". She gets shot while trying to see what the outside world is like, and ends up merging with an artificial alien life form, causing her to split into White Alice and Dark Alice. As Dark Alice, she first tries to get the Jabberwock to destroy all of New York, ultimately aiming to get the city blown up by a nuclear bomb. When that doesn't work, she possesses Katsumi, takes over her body, and tries again to destroy the world with her power. She only stops when Katsumi and Ryo offer her love, which was all she ever actually wanted.
In Axis Powers Hetalia, Russia went from being a tiny, weak country under the control of the Mongols and constantly picked on by bigger nations, to being much larger and stronger. This would not be so bad, except that years of revolution and bloodshed mean have left him a Psychopathic Manchild who terrifies pretty much all of the other nations he runs into. Exactly how bad he actually is is uncertain. He's shown snapping and firing on a crowd of protestors in the Bloody Sunday comic and it's implied that he whipped Lithuania when the Baltics were under his control, but he certain is not the rapetruck he's portrayed as in a lot of fanon.
From England's perspective, America could be seen as this. He went from a young child of a colony to declaring independence as a strong country to being a global superpower.
Shion, and, well, every character from Higurashi: When They Cry becomes this at one point or another. Special points go to Shion becasue she is introduced as a minor side-character rather than a main one. What makes this one so horrifying is that the first portrayal of her is RADICALLY different from the portrayal of her as pure evil in Meagashi-hen.
Dio Brando of JoJo's Bizarre Adventure began life as the very intelligent, but frustrated and nigh psychotically ambitious son of a petty thief in England, 1800s. From there, he quickly became a vampire, a Hero Killer, attempted to raise a zombie army to take over the world, gained the psychic power to freeze time, and even after his death, continues to weigh heavily on the setting. It gets to the point that Dio mentored a From Nobody to Nightmare villain of his own, Father Enrico Pucci, who through him, succeeded in rewriting the entire universe!
Bleach: Done in the Gotei 13Invasion Arc. Ouko Yushima began as a very weak and pathetic shinigami who didn't have the strength to fight in battle or master his zanpakutou. He therefore decided to split himself and his zanpakutou apart into two mod-soul beings that could master his powers. The one mod-soul was the apparent villain Kageroza Inaba and the second mod-soul was the apparent heroine Nozomi Kujou. Kageroza could manipulate space-time, bring the dead back to life and was so powerful he could defeat Ichigo and Yamamoto at the same time. Nozomi could absorb reiatsu, steal the powers of others, combine them with her own power and use it as super-powerful attack, and heal better than even Orihime, she was so powerful she could fight Kageroza even when Yamamoto himself could not. Where the mod-soul powers came from is never explained as none of them were abilities of the original zanpakutou, but when the two were recombined back into Yushima, he was able to combine all these powers to destroy the very existence of Soul Society itself. Being an Ineffectual Loner, he was defeated by both The Power of Friendship and The Power of Love.
In Fist of the North Star, the three Rasho, warlords of the Land of Asura, qualify, since only an Asura who has proven himself by killing hundreds of other warriors is worthy of having a name, let alone rising to the level of Rasho.
Midora from Toriko was a nameless baby born for the sole purpose of being fed to livestock. Now he's one of the most powerful beings on the planet who can literally eat everything around him.
Neo as well, big time. He was a pretty weak appetite demon (diminute, to boot) that was mocked and deemed by its peers to be unfit to travel with them. But, after it experienced its first meal, he began eating any animal no matter how big it was. and then began eating planets, stars and the entire blue universe. And after being reborn several times, eventually becoming Acacia's own appetite demon, he's the biggest pillar in the Gourmet Nobility as their ultimate goal is to breed it in order to seal it once and for all.
Pretty much any Puella Magi in any version of Puella Magi Madoka Magica could qualify; most of them were ordinary girls upon making their contract, and many of those were so desperate they needed the wish. Upon making the contract, they become powerful magical warriors who tend to treat the rest of humanity as livestock. When they become Witches, they almost universally qualify; particularly the apocalyptically powerful Kreimhild Gretchen, born from someone who described herself as not having any real talents. Homura is a particular example; as we learn, before the series, she was a meek and flustered Ill Girl who seemed fairly weak even after contracting. By the end of The Movie, she's a God of Evil.
Seto Kaiba from the original Yu-Gi-Oh! was, along with his brother, a penniless orphan until he won a bet with billionaire Gozaburo Kaiba and was adopted by him. He then proceeded to take over KaibaCorp in a hostile takeover, remake it as a totally different company, and before his eighteenth birthday, become, along with Pegasus, one of the two biggest names in Duel Monsters. Of course the "nightmare" part comes in with how ruthless and cruel he could be early on, being almost as bad a corporate tycoon as his father (and in the manga version, trying to murder Yugi and his friends in the "Death-T" complex). He toned down a little later, but was still a man to be feared.
In the Memory World arc, Thief King Bakura would have been a perfectly ordinary peasant living in a rural village, if it wasn't for his village being slaughtered under the orders of the previous pharaoh in order to create the Millennium Items, leaving him as the Sole Survivor who witnessed the whole thing, leading to a Roaring Rampage of Revenge that culminated in an attempt to summon Zorc Necrophades, part of his soul merging with part of Zorc, being imprisoned in the Millennium Ring, and becoming the Greater-Scope Villain for the entire series.
Jack Atlas was just "Satellite Scum" before Team Satisfaction folded, and Jeagar offered him a chance to escape. One act of theft and betrayal later, he was on his way to being the King of the Riding Duels, and one of the most intimidating opponents on the circuit. He mellowed a little after losing to Yusei (and a lot later on) but the past remains.
Kiryu was a far darker nightmare. Also a nobody from Satellite, he was the reason Team Satisfaction folded, when he turned to violent attacks on security, leading to his arrest, death after mistreatment in prison, and rebirth as a Dark Signer.
Every one of the android villains from the Bad Future of season two - Aporia, Paradox, Bruno/Antinomy, and Z-One himself - were nothing but four common soldiers in the ragtag army of civilians who were brave enough to try fight the horde of Meklords; they were simply the only ones lucky enough to survive. Unfortunately, their attempt to Set Right What Once Went Wrong turned them into Well Intentioned Extremists who caused Zero Reverse and almost made it worse.
General Esdeath from Akame ga Kill!, who started out as a normal, if slightly maladjusted, little girl living in an Arctic tribe with her single father. Fifteen years, the death of her father, and a few mass slaughters later, she's the most feared woman in the entire Empire, a One-Man Army that can create glaciers out of thin air, freeze space and time, defeat entire armies of men twenty thousand strong, and knows a lot more about torture than any normal person should. Oh, and did we mention that somewhere along the way she got a demon in her head that (figuratively) tells her to burn things? When Najenda said she was the scariest person in the Empire, she wasn't lying.
Kaneki slowly becomes an Anti-Hero version of this trope. He begins the story as a meek college student, but his life takes a very different turn when he's transformed into a Half-Human Hybrid. After Yamori tortures him, he decides that he will "removeall the weeds" and spends six months becoming one of the most feared Ghouls in Tokyo. Between satisfying his Horror Hunger through cannibalism and slaughtering the members of the Ghoul Restaurant, he becomes something of a mythological figure feared by Ghouls and CCG alike. Our first glimpse of him after the Time Skip is an ominous figure in shadows with one glowing eye, as imagined by a group discussing the infamous "One-Eye".
Yamori was a rather insignificant Ghoul until he was captured and spent time at the Ghoul Detention Center, where an insane Interrogator tortured him. Driven completely insane, he escaped and became the brutal cannibal known as Jason and eventually, one of the captains of Aogiri.
The sequel provides another example: former Plucky Comic Relief Seidou Takizawa, transformed into an incredibly powerful half-Ghoul and completely insane after literally YEARS of rape and torture.
The Devil Is a Part-Timer!: SadaoMaou began life at the very bottom of the demonic peking order until his life was saved by an angel — Emi's mother, Laylah. While he was recuperating, she tutored him in various things to pass the time, mainly politics, diplomacy and the proper use of military power, which he used to rise up through the ranks and become the Demon King, nearly conquering Ente Isla until he was defeated by Emi and forced to retreat to Earth. Now, he plans to work his way up through the ranks at MgRonald's to eventually conquer the Earth.
Ginga Densetsu Weed: The reason why Hougen and Genba were the way they are is because of a bad owner. Locked up in a small kennel with other dogs, the Great Dane brothers had to fight, kill, and even eat dead and dying dogs. When their owner comes back for them, Hougen and Genba (the only survivors) jump on him, kill him, and eat him.
The series in general seems to have a penchant for these, taking guys that just do something rather excessively and giving them superpowers based on it, like the guy who ate too much crab and became a giant crab monster, or the one who shadow-boxed with a light pull cord too much and became a hardy superhuman that could punch buildings into rubble.
Garou, who went from a kid who found it extremely unfair that the bad guys in his cartoons never won despite their effort, to a menace that wiped the floor with everyone that wasn't the protagonist in an effort to become the ultimate villain.
Saitama takes this to new heights. At the beginning of it all, he was just a recently-fired salaryman whose life felt rather meaningless. One battle against the aforementioned crab monster later, he decided he wanted to be a hero no matter what, and started training like hell to achieve it. Three years later, he's a bald engine of destruction that could punch a hole through the world on a very lazy day, and can't hold back enough to make his fights last.
Tengen Toppa Gurren Lagann is the heroic version of this. It starts with a kid drilling tunnels deep underground, who unintentionally sets off a Lensman Arms Race that ends with a galaxy-destroying cosmic brawl for the fate of the universe.
In Devil May Cry: The Animated Series, Sid is a low level demon grunt whom nobody takes seriously and Dante considers Not Worth Killing. Then he manages to acquire artifacts and conduct a ritual that grants him the power of the demon lord Abigail, becoming powerful enough to beat the shit out of Dante. Dante manages to defeat him by using Devil Trigger, then comments that no matter how much power Sid gets, a loser is still a loser, before killing him.
Tomohiko Katsuragi from Sakura Gari once was a smart, ambitious and obsessive teenager who worked as the butler of a rich Tokyo family, then became the clan's personal doctor and one of the lovers of the family's heir. But not before commiting many, many crimes and abusing almost anyone who got in his way. When his much abused wife snaps and the aforementioned heir finds out what Katsuragi did to his Morality Pet, however...
The main character of Overlord was just a salary man with no friends or family, obsessed with a video game his guildmates had all long since quit. Then he gets pulled into a world that operates on that game's rules, in the body of his character, a Level 100 Elder LichEvil Sorcerer, with all his guild's NPC's (Including demons, dark elves, vampires, and more) becoming his fanatically loyalarmy. The New World never new what hit it.