Our Miss Brooks: Mr. Conklin was nothing more than a nuisance and a blowhard during the first three seasons of the TV show. However, when the show was retooled for the fourth season he was turned into a full-on villain who vowed to make Miss Brooks' life miserable and would even try to get her fired. The two went from being frenemies to just plain enemies.
The Affair: Noah loosely bases his new novel on his affair with Alison in Montauk. He paints a pretty unflattering picture of the Lockharts, making them out to be a hardcore organized crime family. Alison herself is also turned into The Vamp.
In the comics, "Deathlok" referred to multiple characters, all of which were heroes. In the show, "the" Deathlok is a basically good person but Trapped in Villainy. "Ragtag" also reveals that the original Deathlok is also The Clairvoyant, the Season 1 Big Bad.
The Clairvoyant is ultimately revealed to be Agent John Garrett, a loyal S.H.I.E.L.D. agent in the comics. This ties in with Captain America: The Winter Soldier, where some agents who were good in the comics are part of the Hydra infiltration of SHIELD. Just like the heroes themselves, the audience no longer knows who to trust.
The 2015 BBC miniseries adaptation of And Then There Were None changed many characters' backstories to show them straight-up murdering their victims instead of their more indirect killings in the book:
Mr. Rogers, instead of withholding the medicine that kept his sick employee's heart condition in check, smothered her with a pillow. He's also a much bigger Jerkass who abuses his wife.
William Blore, instead of committing perjury that sent an innocent man to prison, brutally beat a young gay man to death.
Philip Lombard, instead of simply deserting a tribe of African natives to their death, straight-up killed them for diamonds. However, he's an odd case of both this trope and Adaptational Heroism applying to the same character as he's still more sympathetic overall than his book counterpart with his more genuinely caring treatment of Vera and the omission of some of his more bigoted statements from the book.
Vera Claythorne comes across as more of a sociopath than the guilt-ridden Haunted Heroine her book self was, being shown to have knowingly bid her time while waiting for Cyril to drown and offering to pin the murders on her lover in exchange for the actual murderer sparing her life.
The episode "Dodger" turns a Loveable Rogue from the comics into a ruthless and calculating killer.
Eddie Feyers in the comic was a shady character but basically an ally to Green Arrow, as long as his CIA bosses didn't say otherwise. After Ollie's death, he even became a Parental Substitute to Conner Hawke. His live-action counterpart is the Big Bad of the Season One island flashbacks.
"Blast Radius" introduces Shrapnel as a Mad Bomber, which is a reasonable de-powered version of the character. The adaptational villainy comes in when the group he's part of - which is identified because they regularly use that kind of bomb in their terrorist activities - is called The Movement.
In the comics, Zoom was the identity of Hunter Zolomon, who was a villain in order to make The Flash, then Wally West, a better hero. (Basically, he believes that trauma and hardship make you a better hero, and he's just the guy to provide it. He's also a good case of Even Evil Has Standards.) Zoom, as the Big Bad of season 2, is a frightening monster who is out to kill all people with super speed to make sure he's the only speedster in the multiverse and gets his kicks by being a Hope Crusher to the people. Hunter Zolomon became a serial killer after seeing his father murder his mother and being abandoned at an orphanage when his other family members rejected him and is doing these terrible things for the hell of it. Although this Hunter Zolomon is from Earth-2. Earth-1's Hunter appears to be a normal human being.
One could argue all these examples are a result of the show using this trope on the entirety of Earth-2. In the comics, it's the parallel universe where Golden Age characters are put and allowed to age and retire as a contrast to the sliding timeline of their Earth-1 counterparts. Here it's an evil counterpart to the main Earth-1 for the most part.
In the comics Warden Wolfe isn't a nice person, and may not even be a good one, but he genuinely wants to keep metacriminals locked up because he sees them as a danger to the city. The TV version is part of Blacksmith's meta trafficking organisation, and is even happy to sell her a hero if he has the opportunity to do so.
Bates Motel: Sam Loomis was a clean-cut, noble man who wanted to work hard to earn a happy life with his fiancé Marion Crane and eventually became the man who'd take down Norman Bates. However, the show's version of Sam is a lying, womanizingscumbag who cheats on his faithful wife with Marion, lying to both women in the process for a cheap thrill. When he eventually gets exposed in his love affair, he plays innocent to try to fool his wife, but she naturally fails to believe him, so Sam goes crawling back to Marion. It turns out that this was intentional because the writers didn't want us feeling sorry for him when he becomes the shower scene victim instead of Marion.
Elementary: Irene Adler doesn't exist, and is a false identity of Jamie Moriarty.
In the comics, both Captain Cold and the Trickster were bank robbers. In The Flash (1990), Captain Cold is an assassin and the Trickster was made into a psychotic and pretty much a dry run for Mark Hamill voicing the Joker.
From Dusk Till Dawn: This happens to Carlos. In the original movie he was merely the brothers contact in Mexico and unaware of the Titty Twisters true nature. In the series hes a full blown vampire.
In the comics, Sarah Essen was a clean cop, but here, she's a reluctant Dirty Cop.
Penguin is more willing to commit bloodshed than other incarnations.
The character of Gerald Crane is a murderous Serial Killer, while in the comics he was a civilian whose worst crime was abandoning his pregnant girlfriend Karen and their unborn son, Jonathan. This is largely because the show borrows more heavily from his characterization in the New 52, in which he is a Mad Scientist who, unlike his TV counterpart, experiments on his son for no reason whatsoever beyond For Science!.
Although she isn't a full blown villain, Barbara Kean isn't quite as nice as she is in the comics. She willingly cheats on Gordon with Montoya (and eventually, the Ogre), and tries to convince Selina that she could use her beauty as a weapon. But perhaps the most striking instance occurs in "Under the Knife", where she begins to establish a firm relationship with the Ogre. When he introduces her to his secret torture room, she isn't the least bit disturbed. In fact, she smiles at him. She's embraced this trope by the season one finale, having killed her own parents and trying to kill Leslie Thompkins.
Hercules does this to the main protagonist's own mother Alcmene, who is turned into an evil high priestess of a Religion of Evil for Hera that sacrifices men on regular basis. She also despises her son for being the product of rape from Zeus — a deity she despises — and tries to kill him by setting snakes on his crib. Hercules' wife Megara is also hit with this trope too due to being made Alcmene's disciple who comes to hate Hercules as well after having a drunken night together with her accusing him of violating her. Along with Alcmene, they conspire to ruin his life together.
Harold Meachum is part of Season 1's Big-Bad Ensemble, while his comic book counterpart played a more passive role being old and crippled, he was by no means a saint there either. Whereas both counterparts are involved in the accident that left Danny orphaned, comics!Harold merely facilitated the death of Wendell Rand because he was in love with his wife while tv!Harold arranged the death of all three Rands because Wendell was about to discover his ties with the Hand.
The most notable example is Collen Wing, who is revealed to be working for the Hand while her comic book counterpart did no such thing. However, she turns against them and becomes a proper hero after realizing the extent of their evill.
Justified: Raylan's father, Arlo Givens, is a ruthless Manipulative Bastard and lifelong criminal with few redeeming qualities. In the original books, he was a coal miner who died of the black lung when Raylan was still a teenager.
Lois & Clark: Mr. Mxyzptlk received this treatment. He was a superpowered imp from the 5th dimension like in the comics, but rather than the relatively harmless trickster as he usually is, here he's a Faux Affably Evil villain who traps the world in a time loop where each time around humanity grows more depressed and pessimistic to make them all cross the Despair Event Horizon and destroy themselves.
Lost Tapes: In all sightings of the Dover Demon, it runs away as soon as it is spotted by witnesses. In the episode of the same name, it behaves like a Serial Killer, trying to trap its victims where it can murder them in sadistic ways.
Mockingbird Lane: Lily Munster's father Grandpa is made Faux Affably Evil and even attempts to kill one of their neighbors, when the Grandpa of the original show was a kindly old man who often used his inventions to benefit his family and their friends.
The Genie from Aladdin. Here, he falls in love with Regina (who is married to King Leopold) and eventually murders him to free Regina from what he believes to be a loveless marriage with King Leopold, the Genie being unaware that Regina has manipulated him the whole time. Even after learning the truth, the Genie remains madly in love with Regina and uses one of his own wishes to always look upon Regina's face, resulting in him becoming trapped in her mirrors (making him a composite of the Genie and the Magic Mirror from Snow White).
Jack the Giant Killer is re-imagined as a selfish treasure hunter who took advantage of a naive and good-hearted giant. However, this is actually an example of an Unbuilt Trope concerning a different folk tale. Jack and the beanstalk is a separate myth from Jack the Giant Killer. Jack from the latter is the hero portrayed in most modern adaptations (though he never ascended a beanstalk) while Jack from the former is considered by some to be a Villain Protagonist who is a petty thief and liar - much like the "hero" proposed in this series. In an interesting twist from the show's formula, audience expectations are slammed by having a character portrayed as s/he was in the original myth, rather than complete deviation.
Regina's abusive and ruthless mother Cora is a Composite Character of the Miller's daughter and the Queen of Hearts. The former was a sympathetic heroine and the latter was Played for Laughs.
They did a really big twist by turning Peter Pan and most of the Lost Boys into the villains. Word of God says they decided to do this because they thought about how messed up a person would have to be if they wanted to remain a kid forever. Interestingly enough, Captain Hook, one of the show's villains, redeemed himself at the end of the second season where Pan is revealed to be the Greater-Scope Villain of one of the Season 2 villains and eventually the first Big Bad of Season 3.
Season 4 turns Little Bo Peep into a villain of the week, making her a witch warlord who turns people into her "sheep" by enslaving them with magic if they can't pay their debts to her.
All twelve of Hans' brothers are minor antagonists, while they were neutral characters in Frozen. While the behavior of at least some of them towards Hans contributed to his villainy (according to Word of God), they were never directly antagonistic towards Anna and Elsa, and his scheme took place without their knowledge or approval. In the series, they attack Arendelle under his leadership.
In Frozen, Prince Hans may have been revealed to be a villain, but his Freudian Excuse of being abused by his brothers made him understandable. Also, he at least showed some humanity, such as asking Elsa to stop the Winter before deciding to kill her. The producers referred to him as a Tragic Villain and his voice actor suggested the possibility of him seeking redemption. The depiction seen in Once throws all of this out of the window. Not only does he outright call Elsa a "monster", he lacks any of his redeeming qualities and the previously mentioned Freudian Excuse, as he's working with his brothers to conquer Arendelle.
Also from Frozen, the Duke of Wesselton receives this treatment. Granted he wasn't exactly a good man (he was a sneaky businessman and a bit of a Fantastic Racist), but he was more of a Red Herring for the true Big Bad and he certainly wasn't an attempted rapist.
Season 5 gives one spot of the Big-Bad Ensemble to King Arthur. In true Once fashion, he's not the nicest or purest knight. The obsession to complete Excalibur led him to commit evil acts such as using Mind Manipulation on his wife or using his knights as pawns.
The Witch is another example from Season 5 Brave arc. In the film, she aided Merida into mending her bond with her mum in their quest. In "The Bear King," she is willing to curse all of Dunbroch and Merida's subjects into bears or bankrupt her kingdom if she does not find the helm as a Secret Test of Character. In addition, she has also cursed Ruby, entrapped as a guard wolf at her Crafty Carver Cottage. Unless she somehow knew that was going to work out for the best and it falls under Omniscient Morality License, too.
From Season 6 is Dr. Jekyll. In Once, he kills the woman he loves, frames Mr. Hyde for it and blames Rumple for manipulating the events that led to the death. Later on, he tries to murder Belle as revenge on Rumple.
A more minor example, from Season 6 flashbacks, is Beowulf, who gets jealous of Rumple for defeating the ogres and tries to ruin his reputation by framing him for murdering villagers.
Prince Wars Gill in Kaizoku Sentai Gokaiger was an Affably Evil spoiled prince that often felt disappointed and woeful whenever his schemes against the Gokaigers failed. In Power Rangers Super Megaforce; Prince Vekar has his petulant qualities largely downplayed and focused more on conquest against the earth and the Power Rangers. His father is rendered in a similar manner: whereas Emperor Akudose Gill was taking over the Zangyack invasion because of disappointment from his son's failures; Emperor Mavro is far more vengeful of his son's defeat, making his desire to defeat the Power Rangers with the Armada more malevolent.
Rita Repulsa lacks all the sympathetic qualities Bandora had, treating her henchmen likes crap and doing evil for the sake of evil. Likewise, Goldar isn't near as loyal to Rita as Grifforzer was to Bandora, switching his loyalty to Lord Zedd at the first chance.
While more of an Anti-Hero than a villain, in the live-action version, Minako Aino is a jaded Broken Bird who treats everyone with derision and deliberately antagonizes Sailor Mars to try to get her to take on a more active leadership role. She defrosts as the series progresses, but dies before she can fully melt.
In the manga, Princess Serenity was a sweet girl who fell off the Despair Event Horizon after Endymion was killed and the Moon Kingdom was destroyed. In this series, Princess Serenity as Princess Sailor Moon is a Yandere for Endymion and the big Plot Twist is that it was her, not Metaria, who destroyed the Moon and Earth Kingdoms with the Silver Crystal. Even worse, she does it again.
In the comics, Chuck Clayton is depicted largely as a good-natured young man with a steady girlfriend and a passion for drawing cartoons. In the series, Chuck is as Jerk Jock and a player, who brags about having gone all the way with Veronica on their first date (which didn't happen) and conspiring with several other jocks in a playbook of their supposed "conquests". After being exposed and losing his scholarship, he gets revenge by crashing Jughead's birthday party and revealing unsavory secrets about most of the main characters. This is a rather jarring departure from the comics given that Chuck was created to add a positive African American male character to the comics.
Jason Blossom is revealed to have been killed in cold blood by his own father, Clifford Blossom. This is quite a departure from the comics, where the Blossom parents, although wealthy, are responsible and down-to-earth, even punishing their daughter Cheryl for her snobby, mean-spirited behavior in an early appearance.
Veronica's father Hiram Lodge is described as a ruthless and corrupt businessman, even capable of harming those in his way. This is also quite different from the comics, where Hiram is level-headed and responsible (albeit exasperated by Archie's clumsiness).
In the comics, the Black Hood is a vigilante who fights criminals and protects people. In the series, he's a Serial Killer.
In the comics, Betty's father Hal Cooper is a kind, mild-mannered man. In the series, he acts kind in public, but at home is abusive and controlling. He's also the Black Hood.
This happened to Salem in Sabrina the Teenage Witch. When Sabrina first appeared in Archie Comics, Salem was little more than a pet cat who occasionally had human thoughts. The series changed him into a more active character, and made him a once-human warlock who had been transformed into a cat as punishment for attempted world conquest. While not a true villain on the show (most of the time) he's usually the resident Jerkass.
Obadiah Hakeswill is a minor example, considering that he's already a violent and depraved Sociopathic Soldier in the books. In the book Sharpe's Enemy, Hakeswill holds off on raping Lady Farthingdale because she claims she's there to pray for her mother; Hakeswill has a... respect for mothers and decides to leave her alone for now. In the same sequence in the adaptation for Sharpe's Enemy, Hakeswill tries to rape her outright and Pot-au-Feu stops him because raping the women will damage the ransom value.
In the original story, Irene Adler has an incriminating picture of herself with the King of Bohemia which she has no intention of using, but keeps as protection against him. In the show, she has multiple pictures of various important people she keeps as insurance, but also hands top secret counter-terrorism information to Moriarty, and blackmails the British government with the photos for millions of pounds to satisfy her own power fantasy.
While in the Holmes canon Moriarty is very much a dangerous villain, he is also a Benevolent Boss and his polite manner is more genuine. In the show, he's far more sociopathic and depraved than his literary counterpart.
Sherlock Holmes: In the adaptation of "The Norwood Builder", the titular character turns out to have murdered a tramp in order to obtain a corpse with which to fake his death. This was because the method used in the Conan Doyle story, involving "dog or rabbit" bones, would simply not be credible to a modern audience, even one aware of the limitations of Victorian forensics.
Sleepy Hollow: In The Legend of Sleepy Hollow, Abraham is the local hero who played a prank on the obnoxious Ichabod Crane by pretending to be the Headless Horseman. The show's version of Abraham is the Headless Horseman, with downright murderous intentions for Crane.
This version of Bizarro is less of a confused Harmless Villain and more of a dangerously intelligent one.
Mr. Mxyzptlk isn't a funny omnipotent imp, but more of a smug psychopath who likes to use his Mind Control to induce Squick and tries to stab Chloe for a deal with Lex Luthor.
Lana Lang is also turned into a much shadier character, although that may well have been unintentional; the show's creators seemed to think she was all but a Purity Sue to the very end.
The comic book incarnation of Zor-El sent his daughter, Kara, to Earth as a protector for her cousin, though at one point he was wrongly believed to have been a villain who sent his daughter as an assassin. The series made him an outright villain who sent his daughter to Earth as part of a plot to have himself resurrected as a conqueror.
Supergirl's Starter Villain is Vartox, who in the comics was a hero and occasional associate of Superman.
Jemm, Son of Saturn, appears in a later episode as a would-be conqueror. In the comics, he's a pacifist hero. He was in opposition to Superman during the New Krypton storyline, but it was more complicated than him being a villain.
Red Tornado is a classic case of A.I. Is a Crapshoot as opposed to the hero he is in the comics. It's theorized that the next version could be heroic, but as of mid-season 2, it hasn't happened yet.
Siobhan Smythe very quickly embraces the role of a supervillain when she discovers her Silver Banshee powers, and hates both Kara and Supergirl. Even before she becomes a villain, she was a jerk. In the comics, Siobhan is a nice girl and quickly befriended Supergirl when they met, even becoming roommates with her. When she becomes Silver Banshee, she uses the powers for good. (Now, the previous Silver Banshee was straight-up evil in the comics.)
In the case of a villain getting worse, Morgan Edge in the comics was a ruthless media mogul with Intergang connections, so yeah, not a very nice guy. However, this version is Lex Luthor-grade evil. (He seems to be a foil for the first character presented as the Lex stand-in, Max Lord. Lord was a Well-Intentioned Extremist who always stopped short of the worst he was suspected of doing and helped the heroes in the end. Edge seems like he's the new Lord, but then we get to dig deeper, and... he ain't.)
Agent Liberty was a hero and member of the Justice League in the comics. In the fourth season, this version of him is the founder and figurehead of the Children of Liberty, a hate group that supports a human-first world order. (The Sons of Liberty were an evil conspiracy in the comics, but when Agent Liberty learned the truth about them, he quit.)
Super Gran: In Forrest Wilson's books, the character Tub, while initially a somewhat reluctant henchman to Campbell, becomes a good guy in later books following a HeelFace Turn. In the TV show, he is a Card-Carrying Villain who goes along with Campbell's plans unquestioningly.
Stargate SG-1 does this to nearly every polytheistic pantheon, as the Goa'uld are Always Chaotic Evil and named after (or inspired; it's never made clear) most early human religions. The most obvious example is Anubis, the evilest Goa'uld by a huge margin who shares a name with a good god in Egyptian Mythology.
The real villain isn't the Affably Evil Silver; it's Squire Trelawney, who plots to cheat Jim and Dr. Livesey out of their share of the treasure, arranges for Mrs. Hawkins to be thrown out of the inn while they're away, has Mr. Arrow executed, and eventually suffers a Karmic Death by diving after the treasure when Jim throws it overboard. Not much like the excitable but well-meaning "most liberal of men" in the book.
Tom Redruth, Trelawney's gamekeeper, gets this, too - in the book, he is an elderly man who accompanies Trelawney to the island and is killed by the mutineers. In the series, he is Trelawney's vicious enforcer back in England.
The usually tecnica luchadora Dark Angel was an antagonist to La Pao on Curva Directa. What was stranger was that it was a Monterrey area talk show, where her best known tecnica work was done.
The Twilight Zone (2019): "Not All Men" was loosely based on "The Screwfly Solution", yet here the story reveals there is no outside influence-it just gives men an excuse to be violent (save one who resists the urge).