Originally, Colonel Moran was going be this in Children of Time, despite the fact that he commits as many despicableacts as his boss, Professor Moriarty, does. In the end, however, the authors discovered that he really truly crossed the Moral Event Horizon several times over, and there was just no way it would be fair to the characters or to the audience to let him live.
Played with in the case of the plasmavore, however, who undergoes something of a High-HeelFace Turn. (She's an alien vampire and first entered the series by going on a killing spree throughout Paris, though out of the necessity of needing to feed on blood to survive, since she can starve to death.)
Death Note Equestria has Ace and Colgate discuss this trope at one point: The Kira Neutrality Movement's plan to remove Kira's motivation to kill by being harsher on criminals doesn't take into account a way to actually capture Kira, letting her get off scott free with mass murder. Ace finds this an acceptable loss; Colgate disagrees.
The Hasbroverse turns Cobra Commander into one — according to a blog post by the author, when Serpentor had Cobra attack the Decepticons, the Commander panicked, grabbed as many bank books/PIN numbers as he could, hopped on a hydrofoil with a handful of loyal soldiers (who probably count too by extension), and fled Cobra Island. Because of this, he not only escaped the resulting massacre, but now lives like a king in upstate New York.
Discussed for Samantha Shepard, who killed millions during her Heroic BSoD in Fractured. So far absolutely nothing has been done as punishment. If anything, the opposite occurred as vast amounts of resources were poured into rebuilding her!
Sarah seems to have gotten off pretty light as well—however many she killed has been implied to be far in excess of what Shepard managed. She remains unpunished for two reasons, the first of which revolves around the stakes and the second being, "How do you actually punish a Physical God?"
Admiral Nimitz does not like that around Pandora, bombarding civilians with nuclear weapons seems to be not only normal but encouraged as a means to punish an errant corporation. For all the talk about how bad this is by Trans-Galactic Republic moral standards, nothing happens to Torgue or the Maliwans.
Jackie Jakobs captured and tortured Vault Hunters and members of Shepard's squad. Given that she was a bit messedup it makes sense for her characterization at the time, but she still experiences no repercussions even after her HeelFace Turn.
Averted for the Illusive Man, whose plans to establish human dominance fall apart in a way that results in his death.
Downplayed for the Citadel Council, who blatantly sentenced whole sections of the galaxy to death but simply live their lives as unremarkable civilians aboard Ultimatum afterward.
In The Prayer Warriors, which features Percy Jackson, Harry Potter and the Carter Kane Chronicles, Voldemort, despite being both an enemy to the Prayer Warriors and Hogwarts, is forgotten about after one attempt to blackmail Draco into killing Michael by threatening to kill Ebony. So far, the Prayer Warriors have also succeeded at killing everyone who disagrees with them; while they suffer casualties in the process, their dead quickly return to life, and any defeats they face are typically, at worst for them, mere setbacks.
The fic Auld Lang Syne basically ends with one depending on the readers perspective on Dexter; after receiving a large inheritance from a previously-unknown aunt, Dexter decides to move to Argentina with Debra and Harrison, his need to kill now replaced by his relationship with Debra, a later flash-forward showing that Dexter and Debra now have a daughter and Dexter makes money writing true crime novels.
At least as of the end of the Savior of Demons three-part Arcos Special, Magnificent BastardRatsura hasn't gotten the comeuppance he sorely deserves, and nobody suspects him as having a hand in the Saiyans' attack.
Littlefinger from A Caged Songbird. Despite starting the entire war, he's rewarded for supporting the new regime with a seat on the Small Council and his titles he accumulated before leaving for the Vale are left intact.
In The Saga of Avatar Korra, Korra herself is a zig-zagged example of a Karma Houdini. In this story, she was kidnapped and raised by the Red Lotus to become a weapon for them. During the end of Book One, she is assigned the task to assassinate Unalaq and his family. While she ultimately decides not to and ends up fighting against the Red Lotus, she inflicted numerous injuries on Unalaq and his family which hospitalized them. She ends up getting horribly scarred by the Red Lotus because of her decision, and she manages to deliver some Laser-Guided Karma to them in return. At the end of Book One and the beginning of Book Two, Tenzin and other officials plan on preventing her from facing legal consequences for her actions due to her mitigating circumstances. Ironically, she is one of the few examples that believes she probably does deserve punishment.
In Lincoln is Done, Lola is punished for disobeying her parents but all the other sisters except Luna did too and they're not punished. Lincoln is also not punished for running away and being mean to his sisters and punching Lola.
In Daycare Scare, the fic's O.C. character, Leo who is the "second Loud brother" and much younger and more prone to crying than Lily, cries due to being in a new environment, in this case being Lily's daycare and thinking that the other Loud siblings expect Lily don't care about him anymore. Lily hugs her brother to clam him down and gives him her pacifier that thankfully sends him to sleep his carseat. But suddenly shortly afterwards, a bully around Lily's age named Billy steals Leo's pacifier and this leads into a fight between Billy and Lily when the latter tries to defend Leo. Billy then blames all of this on Lily to the teacher that results her getting sent to the time out chair for around five minutes, Leo to continue crying and Billy to get completely away with everything he caused. This also causes the main story of the fanfic to happen where Lori stays home to look after Leo and Lily after the former's traumatic experience there.
In Perfection Is Overrated, The Advisor/The Avatar, who created the SUEs (each of whom has tried to kill the Himes and reshape the world as they see fit), never answers for his actions, although Natsuki does (unsuccessfully) try to kill him. The Avatar points out that the sense of futility they feel in trying to make him answer for his actions is the same experienced by those who try to go up against a Mary Sue who, unlike the SUEs, is not on a relatively level playing field with the canon cast.
Earth and Sky: The Flim-Flam brothers' Sycophantic Servant Otto Bomb, who willingly aided them in all their illegal activities in the story, eventually ditches them after they decide to continue the Pegalthon even after being disqualified and ordered arrested. Despite a run in with the Royal Guard, Otto evades capture and is last seen living comfortably in Mexicolt with a beautiful mare, though he's at least reformed a bit by turning his talent for explosives into a lucrative fireworks business.
All things considered, Chrysalis turns out okay too, as she sets herself up as a beloved foal actress in Los Pegasus in order to replenish her power.
Nicely averted with Blueblood. After Chrysalis's impersonation of his wife is foiled, it at first looks like he'll be free of consequences from his role in that and his hateful marriage... for about two minutes, until Fancypants shows up and begins his Humiliation Conga.
In The Nuptialverse, it's established that, unlike canon, Trixie didn't have a Heel Realization after the events of "Magic Duel". Instead, still left half-mad by the Alicorn Amulet, she flees into the Everfree Forest, where she's eluded capture ever since. When she makes her move in Direction, she's finally defeated, captured, and purged of its influence, making her become The Atoner.
Zig-zagged in Vinyl and Octavia Duel Destiny. The main villain, Giovanni is killed at the end, but The Chessmaster gets away with everything they've done, with nopony even realising that they were behind the events of the past two fics.
In A Great and Powerful Heart, aside from a threatening speech from Trixie, Sheriff Brass and the residents of Promise got away with abandoning two foals to their death out of cowardice and racism. Subverted in The Great Alicorn Hunt when Trixie and Jasper expose Brass and his goons in Filli Terram for their crimes, and Celestia orders Promise's to stay under her jurisdiction until it can be reformed.
Subverted. The titular character from Evil Belle isn't really hurting anyone (though she thinks she is), but she makes it clear that she does have malicious intents. But other than the humiliation of her plans failing, she hasn't received any consequences.
A Diplomaticvisit: Balanced Meal essentially got away with murder, harassing the griffon chef Gravon until he could come up with a trumped-up excuse to get him arrested and sent to prison, where Gravon died before Celestia could free him. Balanced went on to found a notorious ponies-first hate group, and despite everything, was never properly brought to justice.
Rayquaza a.k.a. The Ghost King in Latias' Journey. He starts the story by massacring everyone in Altomare note Since Altomare is a Venic Expy, probably around 800,000 people, because Latias turned down his romantic gestures, and then attempts to murder her crush. He eventually apologizes, not because he murdered so many people, but because he made Latias upset. That said, she not only forgives the Ghost King, but also reciprocates his love by only halfway through the story. The author gives a justification that it was just Kyogre's subconscious suggestions that made Rayquaza do it, but this falls apart since comes at the end of the whole story, meaning Latias had forgiven and romanced him when she believed him to be responsible.
The Key to a Successful Interview: While Kasumi is able to defeat the up-until-then-Invincible Villain that was Shampoo and manages to force her to cool it with the sociopathic tactics and stop trying to get rid of Akane, Shampoo is never arrested for her crime of stealing government property and using said property to cause the worst (In-Universe) school shooting in Japanese history (even if nobody was killed), and she decides to (forcefully) become friends with the rest of the cast, saying that Defeat Means Friendship. Understandably, the rest of the cast remain apprehensive of her.
In The Adventure Through Runescape, the Demon Clan escape from prison and pull a Karma Houdini in the last few chapters of the story. The same also applies to the Big Bad's right-hand man, though this is meant to be a Sequel Hook.
White Sheep (RWBY): Discussed. Salem has terrorized the human race for thousands of years; while the full extent of her crimes is unknown, at absolute minimum she used her infinite army of monsters to trap people in small bastions of civilization and killed anyone who tried to strike out into the wilds. Until about twenty years before the start of the story, when she fell in love, started a family, and decided there was no need to destroy humanity after all. Once the world as a whole learns of her existence, it's pointed out several times that a lot of people want to see her punished for her crimes—but her children simply will not allow that to happen. And they've still got that infinite army of monsters. Thankfully, the world leaders are pragmatic enough to accept one criminal (even a terrible one) getting off completely unpunished is a small price to pay for the end of a Forever War.
Pretty much none of the major villains of Sonic X: Dark Chaos are punished for their actions. Maledict pardons the heroes and leaves with his armies in tow, Jesus and the Angels call off their invasion, and Tsali and the Metarex leaders decide to end the conflict and go their separate ways.
After starting a multi-dimensional war and nearly enslaving the universe in the second arc of Hellsister Trilogy, Darkseid fakes his death and escapes to build his strength up back again and put in motion his next scheme, unbothered by his enemies.
In Kara of Rokyn, Nasthalthia Luthor aids and abets her uncle Lex Luthors final scheme which nearly kills who-knows-how-many persons, and then she gets away scot free.
Atonement: Zig-Zagged by the villains Tether converts. Examples:
Purity/Kayden and Rune/Cassie both get off pretty lightly, considering they're formerly members of a Nazi organization, but Kayden did help the Protectorate stop Coil, and Cassie is still just a teenager.
Riley deconstructs this trope. Many people want her to reform for her actions as Bonesaw, despite all the good she's been doing after her HeelFace Turn, and she suffers from a lot of angst over it. Most of the criticism goes away after she helps Madison kill Jack Slash, but it never fades entirely.
Emma never actually faces any punishment for accidentally killing Taylor- Madison at least gets a good deal of community service as punishment. She feels utterly worthless for what she's done (and still suffers from crippling self-esteem issues as a result even many story arcs later) but never actually gets any corporeal punishment.
At the end of Shadowchasers: Torment, most of the villains are arrested or killed. However, the Three, the true rulers of the Cult of Tharizdun who orchestrated the whole plot, remain at large, and at the time of writing, have yet to appear again in the franchise.
It's assumed that most villains in Shadowchasers: Ascension are horribly punished by Mephistopheles for disobeying orders, and a few are killed before the climax; however, Ibza not only gets away with everything he contributes to, he receives a promotion for remaining loyal and betraying his master's treachery. The heroes express how much the distorted politics of Hell sicken them.