Delenn is the Minbari ambassador to the humans and completely devoted to improving the relationship between the two races and always the first to defend humans against criticism. In one episode it is revealed that when the first contact resulted in a misunderstanding that made the humans open fire and got the Minbari Supreme Leader killed, she was the one who gave the order to Kill All Humans in response.
Some of the blame belongs to her colleagues though. Anybody should have known better than to give such a decision to a distraught young woman cradling the dead body of her mentor and first love.
Maybe they did know better: maybe the Warrior Caste wanted to increase its prestige and knew it was a chance to Manipulate her.
She was the deciding vote on the otherwise split council. Essentially, picture the US Vice President. She was obligated to cast the vote that decided what would be done. The fact that she loved Dukhat was irrelevant.
Londo is this toward the end.
Or not. By that time he seems to have given up on his own atonement and was instead working to save his people believing he was damned whatever happened. Which is in its own way rather a heroic if gloomy thought but not quite the same thing.
The tie-in novel covering Londo's ultimate fate (official canon) does state that Londo achieved redemption in the eyes of his people, if not his own.
G'Kar also receives redemption, but has the (mis)fortune to achieve it in his own lifetime and among his own people, becoming revered almost as a living god as a result.
Kurt Bunker—a neo-nazi covered in swastica tattoos appears on the Banshee Police station doorstep to apply for a deputy job, much to heroes' surpise. When dealing with police officers or civilians Kurt is extremely polite and respectful, reserved in his words and quick to follow superiors' orders. Whenever he introduces himself to a new character he apologeticly begins with: "I understand, that my physical appearence may be unsettling...". However when he catches as much as a wiff of his former neo-nazi "brothers" he becomes extremely tense and uses any chance to confront and take them down. He aknowledges that he has committed many atrocities in the name of his "brothers" and wishes to right the wrongs of his past by eliminating the organisation.
Siobhan's ex-husband. He comes back, apologetic and sober. Subverted when it turns out he's not as sober as he claims. He tries to get back into her life after she repeatedly tells him no, then turns violent and angry when she rejects him.
Battlestar Galactica (2003): Caprica Six fits this trope to a T: after seducing Gaius Baltar and conducting the bit of cybernetic sabotage required to render the Colonial Fleet so much space junk and led to the deaths of all but 50,000 of 50 billion people, her visions of Baltar (whom she decides she had loved) make her want to make up for the crap she had done to the humans. Her success is... variable.
Mitchell tries this and he ended up slipping too much due to his Horror Hunger, resulting in murdering 20 people on a tube carriage. After trying to atone during the most of the next season, he realises he never will be able to overcome his blood addiction and his best friend George stakes him to prevent him being used as an "attack dog" by the Old Ones in an extremely tearjerking moment
Also applies to Hal, once an even worse vampire than Mitchell, who has managed to not kill anyone for 55 years, and manages his Horror Hunger by withdrawing from society and obsessively sticking to routine. The Big Bad of the season revealed that this is a cyclical thing for Hal where he becomes The Atoner for a few decades, ends up going Off the Wagon for some time and then tries to find a new way to atone. Whether he will really succeed this time remains to be seen.
Bones: Seeley Booth is looking to save around 50 lives to make up for the 50 he took as a Sniper.
Breaking Bad: By the episode "Blood Money", it appears that Jesse Pinkman has become one, as he tries to give away the millions of dollars in blood money.
In the series finale "Felina", Walter White becomes a very dark and interesting variation. He finally comes to terms with his mistakes and sins and does everything he can to fix as much as he can, getting money for his children, getting Skyler a potential deal with the DEA by pointing her to the bodies of Hank and Gomez, wiping out the Nazi gang and Lydia and eventually saving Jesse before dying. However, he is still completely unapologetic about his sins and even hints that he would do it all again if given the choice.
Angel too, particularly the season 3 episode "Amends."
Andrew tries to become this in Season 7, after his attempt to play Goodfellas with Warren and Jonathan and take over Sunnydale went horribly, horribly wrong and the first evil forces him to kill Jonathan. He means well, but has some trouble fitting in with the good guys due to his general social awkwardness.
Jonathan tries, but he never gets the chance. Before that, he was perfectly willing to go to jail and accept responsibility for his crimes, only running out of fear of Willow.
Castle: Captain Roy Montgomery, as revealed in the season 3 finale. He was a Dirty Cop at the start of his career and ultimately became embroiled in the conspiracy that ended in the death of Johanna Beckett, Kate Beckett's mother. He alone of the three officers involved had a change of heart, devoting himself to being the best damn cop he possibly could, which included mentoring Beckett, "the finest homicide I've ever trained, bar none." He finally gets a Redemption Equals Death scene when he dies taking down a five-man hit squad that was coming after Beckett.
Columbo: In one episode, Lieutenant Columbo claims to have joined the police department as a way to atone for his rowdy past. Apparently, when he was younger he and his friends would stick potatoes in other people's exhaust vents so the cars wouldn't start.
The Ninth to Eleventh Doctors. After genocidally ending the Time War, the Doctor becomes an intermittent pacifist who won't let anyone else commit genocide. The 9th Doctor has it the most, since it was his immediate predecessor who committed the double genocide, but the others had to deal with it also. The 10th handled it the worst, going off the rails after being left too long without adult supervision.
"Gridlock": After the events of "New Earth", Novice Hame was sentenced to serve as the Face of Boe's nurse, and has been working to atone for her involvement in the Sisters of Plenitude's scheme using clones as lab rats.
"Journey's End": Dalek Caan became this after travelling unshielded through the Time Vortex and seeing every single atrocity his species had ever committed. Made much better because Daleks were designed specifically to be unable to repent and seek to atone.
River Song. She was raised as an assassin to kill the Doctor by a Religion of Evil, and after falling in love with him, decided to try and be good.
This figures in the plotline of "A Town Called Mercy", as Jex, a scientist who turned members of his race into cyborg warriors, became a country doctor during the 1860s to make up for his crimes. The Doctor called him out on it. The Doctor is often cast as the Atoner as well, especially after it was revealed that he destroyed his own people to end the Time War after they had become genocidal. Oddly enough, in "A Town Called Mercy", his own Atoner tendencies are called into question by Amy:
The Doctor: No, today I honor the victims first. His, the Master's, the Daleks'. All the people that died because of my mercy! Amy: See, this is what happens when you travel alone for too long.
Downton Abbey: There is Bates. Carson is also, slightly, but it's played for a laugh at his expense and own melodrama.
Also O'Brien up to a point at least. After she causes Cora's miscarriage, she tones down her earlier nastiness a great deal.
The Family: Hank. He loathes the pedophilic urges which caused him to expose himself to a boy, voluntarily takes medication to suppress them, and was careful not to be alone with underage boys before he began taking it.
Firefly: While never explicitly stated in the show, it is hinted that Shepherd Book is an Atoner. The Shepherd's Tale elaborates on what he's atoning for.
Jaime is trying his best to reform by saving Brienne, sending her to protect the Stark girls to fulfill his vow, saving Tyrion from execution, and once again aspiring to be a dutiful Kingsguard no matter how soiled his reputation. Later, he insists on sailing to Dorne on a dangerous mission himself to protect his 'niece' Myrcella from retaliation for the death of Oberyn Martell due to his guilt over releasing his brother, who went on to kill their father.
Barristan swears himself to Daenerys' Queensguard to atone for failing her elder brother Rhaegar Targaryen and accepting King Robert's pardon.
Upon his reintroduction in "The Wars to Come", Lancel Lannister has become very pious and joined a mendicant movement to repent for his past sins.
Robett Glover is clearly regretful of not having supported House Stark before, so he seeks to make amends for it.
Robett:[addressing Jon] I did not fight beside you on the field of battle, and I will regret that to my dying day. A man can only admit when he was wrong, and ask for forgiveness.
Sandor "The Hound" Clegane has been known to kill for the slightest reason. He runs down and kills Mycah, Arya's friend, and robs a devout father and his daughter who gave him hospitality, claiming that they would die in wintertime anyway. But after being beaten by lady Brienne and left for dead by Arya, he gained a new, if somewhat less cynical view on life, after being nursed by a band who worships the Seven Gods. After they are murdered, and he avenges their deaths, he promises to only kill when he needs to, and when he and his companions stumble into the house he robbed, he buries the dead father and daughter. While he does his best to hide it, Thoros sees right through his facade when he's burying them.
After Robert's Hunting "Accident", he tries to make amends and asks Ned to stop the attack on Daenerys Targaryen. Only his request comes too late.
Glee: Sebastian Smythe, the main Big Bad for the first half of the third season, shows signs of becoming this as of "On My Way" after Karofsky's attempted suicide.
Hand of God: KD was once a criminal, but got religion in prison and repented from his past. However, he also comes to believe Pernell is God's chosen servant, and starts killing at his behest (though since he thinks God orders it, that's a good thing in his mind).
Have Gun Will Travel: Paladin's backstory makes him this along with an interesting spin on Redeeming Replacement. He was hired to challenge a man named Smoke who he believed to be a villain terrorizing a town. Smoke sarcastically referred to him as a paladin during their gunfight, and the future Paladin fatally wounded him, learning too late that Smoke was defending the town and the villain was his employer. Thus, he decided to don Smoke's costume and do good in that guise (starting with killing his treacherous employer).
Darius had once been a powerful general whose armies were ready to march over all of Europe. On the steps of Paris, Darius took the head of an Immortal who had been a holy man and seemed to absorb the man's goodness. This caused him to disband his armies and spend the next thousand years as a priest.
Duncan is convinced his old adversary Kirin is up to something as leader of a cult as the man has spent centuries only out for himself. They last clashed in 1970s Cambodia where Kirin refused to help Duncan save some children because he was running a drug deal. However, Kirin confesses to Duncan that when he discovered those children brutally massacred by the Khmer Rogue, he was so overcome with guilt that he's dedicated his life since to making up for his past.
Duncan himself is often pushed to fight an Immortal to make up for failing someone in the past or his own occasional dark moves.
In the Law & Order: Criminal Intent episode Amends, Alex learns that the wrong man was convicted for her husband's murder. The real killer was whisked away before he could face consequences, but he felt so guilty about what he'd done that he gave up the criminal life and decided to become a doctor in an attempt to somehow make up for what he had done.
Leverage: Eliot Spencer. The other team members were all non-violent Con Artists, Playful Hackers, and Classy Cat Burglars and seem to view their previous careers as hobbies, but Eliot was a ruthless mercenary and deeply regrets the things he did. Word of God states that Eliot doesn't truly believe atonement is possible for him, and looks instead to prevent other people from going down the same road he did.
Lexx: "In the light universe, I have been darkness. Perhaps in the dark zone, I will be light."
Subverted Trope with Mr. Eko, who, as a former child soldier and later drug trafficker and ruthless killer, seems to be a clear-cut Atoner—until he is finally revealed to be utterly unrepentant, considering his past actions necessary first to save his brother and then to survive the bloody lifestyle he willingly took upon himself in doing so. Then he gets killed by a giant black smoke-tentacle.
Richard Alpert started as Atoner, which was his reason for gaining immortality from Jacob. After he accidentally killed a doctor for not giving medicine to his dying wife, a priest told him during confession that he will never gain redemption for his sin. Upon meeting Jacob, Richard was offered a job and a gift: while Jacob could not resurrect his wife or absolve him from all his sins, he granted him immortality so that he atone for his actions.
Ben finally became an Atoner by the end of the series, experiencing a personal breakthrough, helping Hugo to watch over the island in life, and staying behind in the flash sideways feeling he was not yet ready to move on.
This is essentially the entire plot of My Name Is Earl. The titular character is a petty criminal and all-around Jerkass who one day learns of the concept of karma. This causes him to deduce that the reason his life is so crappy is because he does nothing but crappy things, so he writes a Long List of every bad thing he can remember doing in his life and sets out to make up for every last one of them. While initially he was just doing it to get bad things to stop happening to him, he quickly learns that Good Feels Good and gradually becomes a legitimately good person.
Mr. Finch. He created a software program that predicts grand terrorist incidents as well as "smaller" crimes (namely murder), but made it to where the program dumped all of the data on those "smaller" crimes every 24 hours. After a series of incidents where he was made cognizant of how bad an idea that was, he began to seek out someone who could help him prevent one of those "smaller" crimes from happening each day. When he found Reese, he knew he found his man.
Reese is also trying to atone for his years spent as a government assassin. His work caused him to abandon his girlfriend and later prevented him from saving her from her abusive husband.
Over the third season, Finch and The Machine help Root understand the value of "irrelevant" human life, leading her to shift from an amoral hacker/assassin to the Token Evil Teammate to someone who is genuinely regretful of her previous actions.
Under the influence of the Machine Team, Detective Fusco transforms from an alcoholic Dirty Cop to a straitlaced hero, initially because Reese forced him to help out, but then because Good Feels Good.
Even Shaw, a former government assassin who doesn't really feel anything when she kills people, begins to enjoy saving others.
Several of them in Once Upon a Time, most notably Regina, aka The Evil Queen, and Killian Jones, aka Captain Hook, both of whom had legitimately sympathetic backstories, and made a HeelFace Turn due to love, then join the heroes to try and make up for their pasts. Mr. Gold/Rumplestiltskin was also a case of this in season 3 until his own death, and painful resurrection, followed immediately by the death of his son drives him back to being a villain.
The Outer Limits (1995): In "Small Friends", Professor Gene Morton is a convict in his 70s who killed a fellow scientist for trying to steal the credit for his Nanomachines research 15 years earlier. He destroys his chances of being paroled by telling to the parole board that he could not say with certainty that he would not react the same way in similar circumstances. However, it turns out that this was a calculated move as he believes that he deserves to stay imprisoned because he feels so guilty and wishes to atone for his crime.
The Outpost: Talon's mentor, The Smith, makes it clear that part of why he's training her is as a means of atonement for his own past sins. Specifically, the part he played in the massacre of her people.
Power Rangers Jungle Fury: At the end of the series, Jarrod decides to start over at the academy to make amends for his time as Dai Shi's vessel.
Power Rangers RPM: Dr. K only wanted to go outside so she released a computer virus that destroyed the world, and spends the entire show making up for it.
Primeval: A medieval knight who pops through an anomaly and becomes the Monster of the Week turns out to be this. He spent a bunch of time as a mercenary, presumably doing some not-so-nice things, but he's convinced that if he can defeat a dragon (actually a Dracorex, a draconic-looking herbivorous dinosaur), he will have atoned for his crimes.
Michael is a double example. He embarks on his quest to free Linc from Fox River to atone for not appreciating the sacrifices Linc made for him and for thinking he was guilty. Later in the series he attempts to atone for all the deaths his actions have indirectly caused.
Brad Bellick sacrifices his life in order to ensure the survival of the rest of the group. He had spent the enitre series until then trying to chase down and kill the group members.
Agent Mahone agrees to help Michael Schofield in Season 4 to help him destroy the Company. Previously, Mahone had ruthlessly pursued Michael Schofield and had even killed the latter's father.
Revolution: As episode 3 reveals, Miles Matheson did some pretty brutal stuff and was probably more brutal than Sebastian Monroe at first. He executes two men who he believed murdered some travelers despite Monroe's objections. He seems to regret some of his actions which led to his leaving the Republic and him assisting the rebels. In Episode 7, Miles help in saving a group children from being "re-education" by the Monroe militia, because the town's parents were all killed back when Miles was in charge. As Episode 14 reveals, Miles had a protege named Alec Penner until he handed them over to Texas for failing to assassinate the Texan president. As Episode 17 reveals in a flashback, Miles is known as the Butcher of Baltimore, and that he supposedly tortured Rachel to death for tricking him. As the first season finale reveals, when Monroe killed a rebel and his entire family for hurting Miles, Miles realized that Monroe had gone too far. He tried to kill off Monroe, but he couldn't do it. Subsequently, Miles and Nora Clayton left the Monroe Republic.
Scandal: Olivia wants to do right by her clients, but especially the ones who are women caught in infidelity scandals. Those hit a little close to home for her.
Shark: Sebastian Stark from this TV series was a ruthless defence attorney until a client killed his wife shortly after Stark got him acquitted of spousal abuse charges. He turned around and joined the District Attorney's office, using his underhanded legal tactics to put away the types of criminals he used to get off.
Lionel Luthor, the reprogrammed Brainiac 5, and especially Tess Mercer all of shades of this following their respective Heel Face Turns.
Tess Mercer's character arc for the tenth season is basically this. After a HeelFace Turn at the end of season nine, she spends the beginning of season ten mostly staying out of the way, and the rest of the season as the new Watchtower, helping the good guys while clearly hoping that she can at least try to make up for the things she'd done before. By the end of the season it also seems at times like she's trying to atone for her surprise Luthor bloodline.
Teal'c wants very much to atone for his actions as the servant of the Big Bad. This is especially true when he is put on trial for a killing he did as a Mook. Even though the trial is completely unfair, Teal'c refuses to escape and is determined to take the punishment as a way of making it up to one of his victims somehow. Fortunately, the Goa'uld attack the proceeding and Teal'c defends the innocents so wholeheartedly and that his accuser forgives him. Daniel Jackson, in defending Teal'c at the trial, practically has to pull teeth just to get Teal'c to admit anything in his own defense, such as why he chose an old crippled man when he was order to kill one of the crowd as an example (Teal'c knew that the people of the village could escape to tunnels when attacked by the Goa'uld but they wouldn't leave anyone behind, so he killed an old crippled man so the rest of the villagers could survive later.)
Tomin gets like this in The Ark of Truth, after realizing how the Priors are twisting the religion of Origin to excuse genocide. Teal'c gives him some advice on how to live with himself despite the things he's done in the past: he should concentrate on helping other people, fighting so that others can be saved. While he may never achieve personal redemption, "that is the least you can do."
Star Trek: The Next Generation: In the episode "Deja Q", Q is stripped of his powers and made into a normal human who is dumped onto the Enterprise-D. He spends much of the episode trying to learn to be a human. Since he is a jerkass, he has made many enemies, all of whom are searching for him. At the end, he shows he has truly atoned when he steals a shuttle to make a Heroic Sacrifice to keep the Enterprise safe from one of his enemies. The other Q restore his powers, and his normal personality re-asserts itself.
The Vampire Diaries: Stefan. He has been a Ripper on and off for his entire immortal life, but has always realized the errors of his ways and therefore, always tries to atone for his dark past.
Westworld: Teddy can never settle down with Dolores because he first must atone for - actually they didn't bother to program what he felt guilty over. After many years of being motivated by undefined guilt they finally gave him reason for it, his involvement with the murderous Wyatt. This turns out to be based on long-erased memories of helping Dolores/Wyatt in a massacre of Hosts in an attempt to close the park down.
Xena: Warrior Princess: This trope is the show's premise. Xena spends the entire series atoning for her misdeeds, and dies to achieve redemption.