What was Amanda Waller before she became one of the most dangerous and manipulative people in the DC Universe, a person whom even Batman fears? A normal housewife in Chicago with a family—and then her family died (her son was killed in a mugging; her daughter died fighting off someone who wanted to rape her, the neighbors ignored her pleas for help, and Waller's family had to have a closed-casket funeral for her due to the extent of the injuries that killed her; and her husband was killed after avenging their daughter) and she decided to claw out of the pit without any help whatsoever.
Skinner Sweet was just the son of mint farmers who ended up orphaned at early age. He went on to become a notorious outlaw from The Wild West who ended up messing with the wrong people, namely a coven of European Vampires. During a confrontation with them, some vampire blood splashed into his eye and he rose as the first American Vampire, stronger than common vampires, powered by sunlight and immune to all their weaknesses. And one Bad Future shows him as an Evil Overlord ruling a a post-nuclear Earth as The Beast's host.
His first progeny, Pearl Jones, also qualifies though she is a heroic example. Initially a young actress with dreams of stardom, she was offered as prey for a Carpathian coven of vampires. Like many previous victims, her body was dumped in the desert to be forgotten, but alas she clung to life long enough for Sweet to give her an Emergency Transformation and save her life. For the following decade, she pursued the ones responsible, becoming the bane of the Euro-Vamps along with her creator.
Dracula of all people, and also doubled as The Spook. Although his true identity is a mystery, he was widely speculated of being merely a pig farmer in Romania before rising as a vampire instead of Vlad the Impaler. In any event, his bloodline wasn't considered particularly virulent or powerful at the time of his creation and went by largely ignored by the Vassals of the Morning Star until the 1800s when he nearly exterminated every other vampire species in the world, making the Carpathian race the single most populous in the world.
The Authority: Seth Cowie was just some inbred, skinny redneck that one day got picked up by a secret cabal that turned him into a monstrous cybernetic Super-Soldier with literally thousands of superpowers and was deployed specifically to neutralize the titular superhero group. He also became a twisted psychopath with a penchant for hurting people, specially little girls.
The Joker. Whether he was in fact a failing stand-up comedian or a two-bit criminal in the beginning, his One Bad Day turned him from Joe Blue Collar Average into a horrific monstrosity that frightens other villains. Particularly interesting in that he never gained any actual superpowers (other than an incredible resistance or immunity to poisons and toxins), instead deciding it'd be really funny if he concentrated all his efforts to racking up a bodycount that'd make an Evil Overlord blush. In fact, when counting only single, personal acts (not city-destroying, etc.) it's without question the highest body count in the DCU.
In Death of the Family, Batman sends Joker into a Villainous Breakdownby claiming that he knows Joker's real name, reminding Joker that he used to be Nobody before becoming a Nightmare.
In Darkseid War, Batman briefly becomes a New God by sitting on Metron's chair of omniscience. He almost immediately tries to learn the Joker's true name, merely saying "That's impossible!" when given an un-shown answer. This was that the Joker was, in fact, three different people.
Killer Moth was incompetent and weak. Then he made a Deal with the Devil to become "feared," and Charaxes, a man-eating Human/Moth hybrid, was created.
Mr. Freeze was once a cryogenics scientist who just wanted to cure his wife's fatal disease. One nasty accident later and he becomes a vengeful, emotionless supervillain.
The New 52 version of Mr Freeze was changed into always being a monster. He was sickly fascinated with cold since a child, killed his own mother, and the Nora he obsesses about is not related to him in any way. Exceedingly unpopular, the change was dropped like a rock with DC Rebirth.
In his Golden Age debut, Basil Karlo was a horror movie actor who suffered a mental breakdown and started murdering people disguised as the monster Clayface. While dangerous, he was basically your average Scooby-Doo villain and a very small-scale threat. After several years of absence where the Clayface role was taken by three characters with actual powers, Karlo returned to the spotlight by using their DNA to mutate himself into a virtually unstoppable shape-shifting clay elemental, and became one of the most deadly beings in Gotham City.
Poor little Waylon Jones was a boy born in Louisiana with a rare skin condition that made him the target of bullying and abuse. Eventually he snapped and began viciously attacking his tormentors, then joined a carnival where he earned a reputation as sideshow performer. As he aged, his scales hardened, his teeth sharpened, and he became enamored with being feared. Jones soon moved to Gotham City and worked his way through the underworld to become Killer Croc, a living urban legend who has given Gotham's protectors many a reason to fear the dark.
The Penguin was little more than an unpopular child who was teased by other kids because of his appearance. When his mother died, her long illness costing him so much that the bank foreclosed on the family business, he turned to crime, only to be mocked by criminals too. This caused him to find some backbone that he never realized he had, stand up for himself by taking over a criminal gang by force, and eventually become one of Batman's greatest enemies.
The Scarecrow started out as a lanky, bullied bookworm. Wanting to turn the tables, he began a lifelong study of the psychology of fear. Later on, he began experimenting with the physiology of fear, becoming a maker of literal Nightmare Fuel.
For a while, people thought that Scarecrow wasn't much of a threat without his Fear Gas. Then he went out on a killing spree sending the city into the a state of panic causing some of the villains in Arkham to beg Batman not to bring Scarecrow back to the asylum.
Warren White was your typical corrupt white collar criminal who made the foolish mistake of trying to avoid jail time by pleading insanity...in Gotham City. A few months in Arkham and one horrific face-tearing attack later and Mears was the sadistic and wicked Great White Shark.
Batman himself is a rare heroic version of this Trope. One night, a family out on the town takes a wrong turn down an alley. The parents are murdered by a mugger, leaving an eight-year-old boy orphaned and traumatized. It was the kind of random street crime that happens every day in Gotham City. It could have happened to anyone. Except that eight-year-old boy resolved to rise above his trauma, to avenge his parents, and to ensure that something like this never happened to anyone else ever again. After ten or fifteen years of training his mind and body to their peak performances, using his extensive family fortune to design and build custom crimefighting equipment, and using his family name and business contacts to develop a global network of assistants and allies, Bruce Wayne is the finest deductive mind, the most well-equipped vigilante, and the most terrifying nightmare of any criminal in the DCU. (It could be argued that the Waynes were not nobodies by any stretch of the imagination, and it's questionable Batman could exist without the family fortune.)
Ghoul from the Batman Beyond universe was just a regular mook when he started out. After murdering Jake Chill, he became one of Terry's most recurring foes, as well as The Man Behind the Man for several other villains.
One of those villains was Rewire, just a regular teenager with minimal electricity manipulation abilities until Ghoul got a hold of him, gave him a suit of Powered Armor, juiced up his bioelectricity powers and convinced him to kill the mayor of Gotham. Which he succeeded in doing.
This entire trope is essentially the synopsis of Batman Eternal, where a group of low tier, forgettable villains bring Gotham to its knees with the entire plan hinging on the theory that they're so low on Batman's food chain he wouldn't even be able to figure out who could commit a plan of its sheer scale without mistakenly targeting his A-class villains such as Ra's al Ghul or Riddler...and they were right.
Blackest Night reveals that the Entities, the embodiments of Hope, Compassion, Greed, Fear, Willpower, Love, and Rage, were originally the first lifeforms to feel those particular emotions. Parallax, the one responsible for Hal Jordan's Face–Heel Turn which nearly wiped out the universe in Zero Hour... used to be an ordinary little bug.
Which also counts as Fridge Brilliance, as Parallax is the embodiment of Fear; what creature would be more likely to be afraid than an insignificant little bug?
Black Hand was a C-list villain (based on Batman co-creator Bill Finger) who stole energy from the Green Lanterns and based his persona and crimes off of in-jokes and folk sayings. At one point he gave up supervillainy to run a porno theater. He then led the Black Lanterns, and was a primary antagonist in the DC Crisis Crossover, Blackest Night.
Rift, who was a postal worker who lived simultaneously in the DC Universe and the Dakotaverse. When he became Rift, he treated both universes as playthings.
Mister Mordan was a near-invisible member of Doom Patrol foes the Brotherhood of Evil. After undergoing experimentation he became a sanity-draining nihilist (ironically calling himself Mr. Nobody) who, along with his new-formed Brotherhood of Dada, trapped Paris inside a painting and almost unleashed the Fifth Horseman of the Apocalypse. In a later adventure he used Mind Rape and almost became President of the United States.
Abraham Pointe was a creepy nobody photographer stalking a waitress in a Las Vegas casino. Then he got possessed by the Predator, the Entity of Love, and became a superpowered stalker.
The Human Flame from The DCU is a subversion. True, he started out a pathetic D-list villain that gained incredible power during his mini. The subversion is that, even with a huge power boost, he's still pathetic.
Johnny Sorrow was a silent movie star whose career was ruined by the coming of talkies. Turning to crime, he used a device to become intangible until a battle with the JSA caused it to malfunction and suck him into another dimension. There, he was transformed into a twisted being whose face literally kills anyone who gazes upon it and determined to make Earth pay for his condition.
Maxwell Lord was your typical yuppie, ruthless in the field, even wiling to let his boss die to get ahead but trying to save him. He seemed to be a good guy working with the Justice League International and gaining the power to push his will on others. But when Cyborg Superman destroyed Coast City (where his mother lived), Max became convinced that metahumans caused more problems than they solved. Driven by his new crusade Max would become more obsessed with "saving" humanity... which ended up turning him into a mass murdering menace.
John Dee, aka Doctor Destiny, was a minor JLA villain who washed up and got locked away for years... until gaining control of the Materioptikon and becoming a global threat overnight, leading to a showdown with the Lord of Dreams himself.
Dee directly lead to the creation of another threat to reality when his Materioptikon ruby ended up in the hands of a frustrated nobody named Darrin Profitt. After using it to create billions of alternate realities where he explored the outcomes of any decision possible in his quest for power, he comes out the other end as the Red King; a man with contingencies for every Justice Leaguer and all their resources, advanced transforming power armor and over 63 different metahuman powers.
Justice League of America's enemy Dr. Light. Turns out his harmless joke status was caused by an accidental magic lobotomy, and that his original personality was a lot morethreatening and now that he's beaten the effects he's a lot more dangerous. Creepiest bit, as brought on by way of retcon, is that for the years in between the League's been letting teenagers fight him without letting on how depraved he really is. His preference for rape has been so Flanderized to the point that many readers have begun to refer to him as 'Dr. Rape'.
Sinestro was just another inhabitant of his home planet when he was chosen to join the Green Lantern Corps. Rising as one of their greatest members, his obsession with order made him become absolute dictator of his home world. When he was exposed, he was cast out, forging a yellow ring to fight the other Lanterns, setting up Hal Jordan to destroy the Corps and eventually forging his own Yellow Lanterns to spread fear across the universe. And he still insists he's in the right for it all.
Supergirl's enemy Blackstarr in Supergirl Volume 2 was a poor, unlucky child who was taken from her parents when her family arrived at a concentration camp. She survives, though, and after being indoctrinated by Nazis and gaining powers she becomes a villain able to rip apart the space-time continuum.
Dr. Hank Henshaw was basically a gag Expy of Dr. Reed Richards in a forgettable Superman comic in which a group of scientists become Fantastic Four Expies - but don't survive the radiation. Well, Henshaw survives, and becomes the Cyborg Superman, one of Superman's most powerful enemies.
Doomsday, who killedSuperman. It is eventually revealed that he was created through forced evolution on a random, harmless, adorable baby, turning it into a superstrong immortal killing machine.
Then there's Lex Luthor, who grew from a kid in Suicide Slum (or Smallville, depending on when you started reading Superman) to supergenius and probably the single richest, most powerful, and corrupt businessman in the entire DC Universe.
Whatever Happened to the Man of Tomorrow? starts with a number of the Harmless Villains from Superman's Rogues Gallery resurfacing for the first time in years, now much more lethal and dangerous in their methods. Even though he deals with them handily enough (though not before they rack up body counts) Clark begins to worry: "If the nuisances from my past are coming back as killers... what happens when thekillerscome back? This turns out to have been the influence of Mr. Myxlptlk, who in a reverse, reveals that he was never as harmless as he looked; he was just playing the part of a harmless trickster imp, but he's done playing.
Reign of Doomsday: Doomslayer was originally a mindless Doomsday clone created by Lex Luthor to distract and hound the Super-Family while he carried out one of his schemes. Nothing the Supers had not faced and beaten several times before. Then, he gains sentience, extra-knowledge, and an undying hatred towards all living beings which leads him to attempt to blow Earth up.
In Alan Moore's Swamp Thing run, the Floronic Man realizes that with his control over the growth of plants, he can choke the world any time he wants.
Vandal Savage was just an ordinary caveman before he found the meteor that gave him his powers. Had he not, he would have certainly died with no one ever remembering him. Instead he became one of the vilest, cruelest villains in the DC Universe, a man responsible for untold murders and multiple wars. All because he was attracted to a bright light...
Eviless was not even named in her first two appearances, just blending in with generic low level Saturnian slavers. She later goes on to be a major player in the attempt to start a war between Saturn and Earth, invades Paradise Island and founds one of the earliest Villain groups in comics.
Hypnota was a simple stage magician with a small show, who gained powers after being shot in the head and choose to use them to twist the minds of others.
Wonder Woman (1987): Veronica Cale started out life as the destitute daughter of a poor prostitute, and rose to become a world renowned scientist and CEO who makes deals with monsters from ancient myth, murders with abandon, and conspired to start a war that was only averted by the universe getting a retool.
was just a normal little boy named Cobi. Then his planet was invaded by the Star Conquerors. The death of his older brother (which led to Cobi being possessed by the collective pain and rage of his people) granted Cobi Power Born of Madness, which he used to enslave the Star Conquerors. Cobi went from an innocent little boy to an insane Humanoid Abomination Mind Hive carrying the pain and hate of an entire race with complete mastery over a parasitic Hive Mind of Eldritch Abominations.