The episode "Dodger" turns a Loveable Rogue from the comics into a ruthless and calculating killer.
Eddie Fyers 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, a Psycho for Hire under contract from Amanda Waller who believes he's shooting a Chinese plane out of the sky to destabilize the country's economy (when really China White was on board and Waller wanted her killed).
"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 Lonnie Machin was a Well-Intentioned Extremist with a firm moral code (in fact, it's arguable whether he's even a villain or just someone who doesn't see things the same way as Batman). In the series, he's an Ax-Crazy sadist who prompts an Even Evil Has Standards reaction from the Season 4 Big Bad. There's crazy, and then there's so crazy that freaking Damien Darhk refuses to work with you!
Talia al Ghul, much like her depiction in The Dark Knight Rises, is mainly portrayed as a villain rather than as an occasional ally, even being the one who trained Prometheus for revenge on Green Arrow, though she does later help out in season 7 and has made peace in season 8. This is likely due to her more noble traits being transferred to her sister Nyssa.
Emiko Queen, the half-sister of Green Arrow, turnsout to be the season 7 Big Bad, a sharp contrast to her heroic character in the New 52 era of DC, when she is revealed to be a leader of the Ninth Circle, a group she originally opposed, and once was a member of in the comics ... as a Mole.
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, therefore a foil to Barry, who also saw his mother die, but his father wasn't guilty and he was adopted by Joe West. He was actually Wally's Friend on the Force in the comics before being crippled, which lead to his use of the Cosmic Treadmill to become Zoom. Although this Hunter Zolomon is from Earth-2. Earth-1's Hunter appears to be a normal human being.
The first villain of the second season is Atom Smasher, who in the comics was the successor to the Golden Age Atom and a member of Infinity, Inc. and the Justice Society of America. He kills his own Earth-1 self and is the first metahuman sent after Flash by Zoom.
While she's abrasive and kind of a bitch, Doctor Light is a hero in the comics and has been a member of the Justice League for years. In the show, she's a villain and tries to kill Linda Park so that she can replace her and take over her life, being an Earth-2 version of Linda.
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. (Basically, Earth-2 and Earth-3 from the comics are switched. In the comics, Earth-2 was the world of the Golden Age stories and Earth-3 was a Star Trek-style Mirror Universe. In the show, Earth-3 is where Jay Garrick hails from and Earth-2, while not exactly populated by evil versions of the show's full cast a la Star Trek, has many an evil version of a hero, or a good or at least civilian version of a villain.) When Earth-2 was destroyed in the Arrow season 8 premiere, and the multiverse resurrected as a whole in Crisis on Infinite Earths (2019), it lead to a new Earth-2, the setting of Stargirl, which was more accurate to the original comics version.
In the comics, Eliza Harmon/Trajectory Jumped at the Call to be a hero when she gained Super-Speed, and while a member of the Infinity, Inc. formed by Lex Luthor, she was not alligned with him and was swiftly killed in the field. Here, she's a Card-Carrying Villain who uses her speed to hurt people and destroy property For the Evulz. Psycho Serum made her that way, but she didn't get to redeem herself before Phlebotinum Overload let to her running herself right out of existence.
The true identity of Savitar. The Future Flash came back in time to prevent Wally West's deaths, but hardship caused him to become a more violent speedster, one who came to regret what he became upon death. This version is attempting to murder Iris in order for present Barry to become so full of grief that it leads to his existence and seek godhood.
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 organization, and is even happy to sell her a hero if he has the opportunity to do so.
Doctor Light. Yes, again. The post-Crisis universe introduces a new version who has the real identity of Kimiyo Hoshi, like the comic heroine, but is an assassin working for Black Hole.
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 5, it hasn't happened yet, apart from the heroic Earth-X version from Crisis on Earth-X.
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.) He would later be re-interpreted in Superman & Lois as Superman's long lost stepbrother Tal-Rho.
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.)
Dick Malverne in the comics was Supergirl's love interest in Midvale, and a loyal Secret Secret-Keeper. Rick Malverne in the series also grew up with Kara in Midvale and learned her secret, but then kidnapped Alex to force Kara into releasing his murderer father from prison.
In virtually every incarnation, John Henry Irons is rescued by Superman and is inspired by him to become a superhero, creating a set of Powered Armor and becoming the superhero Steel. In this series, John Henry Irons is the Sole Survivor of an invasion of artificial Kryptonians lead by Superman in another universe and he seeks to kill this universe's Superman to prevent a second attack. It's also a case of Death by Adaptation as the main universe's Irons is mysteriously dead.
