Atonement was a huge theme in Buffy the Vampire Slayer because being able to make mistakes and learn from them was part of growing up. The supernatural drama that used the forces of darkness as metaphors for the horrors of adolescence made the traits that formed heroes and villains mercurial, and therefore authentic to real life. Often the characters like Angel, who committed some of the most depraved acts of violence, were capable of profound sacrifice, while some of the characters like Willow, who pledged themselves to do only good, had the capacity to generate unspeakable chaos.
The line between what made a character good or bad was nebulous, which made not only for compelling television, but introduced the opportunity for anyone's redemption. Sometimes characters had moments of redemption, and sometimes their development seemed like one long redemption arc stretched over seven seasons.
Jonathan
Before he became a member of The Trio, Jonathan was a classmate of the Scooby Gang's who like Xander, suffered from feelings of inadequacy and low self-esteem. Though he didn't like being bullied, Jonathan let Warren's domineering personality sway him, until his callous disregard for human life appealed to Jonathan's empathetic and comionate nature.
He tried to set things right by telling Buffy how to defeat Warren, but Willow's homicidal rampage forced him to flee Sunnydale. He was offered protection by the Scooby Gang, but he died before he could warn them about the First Evil (and finally become a member). He notably reached a redemption in his final moments over the Seal of Danzalthar; he wished his high school bullies well, even if they didn't care about him.
Andrew
Like his big brother Tucker, Andrew was able to summon demons, but that's where the similarities ended. Much more of a nerd, he gravitated towards The Trio with Warren and Jonathan because it gave him a false sense of confidence and a fledgling feeling of power.
Unlike Jonathan, Andrew showed little remorse for the The Trio's malevolent escapades, including killing Warren's girlfriend Katrina. In the wake of Warren's death, Andrew and Jonathan ran to Mexico, where Andrew worked for the First Evil, and ended up killing his best friend, which he atoned for by helping the Scoobies at the battle of the Hellmouth despite thinking he wouldn't survive. Thanks to Anya's bravery, though, he did live and later became a Watcher under Giles' tutelage.
Faith
Faith was perhaps one of the most controversial characters in the Buffyverse because her redemption arc was shown in its entirety, from her arrival in Sunnydale with a major chip on her shoulder, to descending into evil at the first signs of feeling rejected by Buffy and the Scooby Gang.
After losing the Mayor and her family, Faith allowed self-loathing to lead her on a path of self-destruction until Angel helped her express her remorse in a way that allowed her to surrender herself to ability and jail time. She only broke out at Angel's time of need, helping the one person who showed her comion. Faith acknowledged that the only way to atone for her evil past was to set aside her mistrust and commit to doing better every day and making the world a better place.
Anya
Though the difficulties in Anya's life, such as fitting into a human world and proving herself to the Scoobies seemed to focus on comic relief, her redemption was nontraditional and therefore unique. After spending thousands of years as a vengeance demon performing acts of atrocity on behalf of betrayed women everywhere, she strove to feel worthy of love.
She fought alongside the Scoobies, helped Xander through his problems, and proved time and again that just like Spike or Angel her violent past didn't define her forever. Once a demon, not always a demon, and very often a brave heroine.
Xander
Xander often let his feelings of inadequacy over the years be the driving force behind his actions, and because of his tendency to not believe his friends' assurances about his value, his low self-confidence caused him to do horrible things, like leave Anya at the altar or continuously betray Buffy like he did with Angelus' re-ensouling.
Unlike many of the Scooby Gang, Xander had no inherent magical abilities, but he stood up to bullies, fought supernatural foes when he could, and after throwing Anya aside tried to make amends by fighting alongside her at the battle of the Hellmouth.
Willow
Comionate, kind, and loyal to her friends, Willow didn't have a lot to atone for until Season 6, until her grief and anger following Tara's death caused her to commit heinous acts of murder and destruction. She didn't care who she hurt, because nothing mattered anymore if she couldn't be with the person she loved.
Even after she was able to resist the temptation of dark magic, her friends didn't trust her, and she had to work hard to return to their good graces. Her magic was useful in defeating the First Evil and saving Sunnydale, but at a great cost to her health. Eventually, she learned how to use magic in a way that didn't cloud her judgment and cause her to hurt the people around her.
Buffy
A hallmark of Buffy's character was placing the needs of others before her own, something that very much went with the Slayer territory. Whether she liked it or not, self-sacrifice was part of being the Chosen One, so when she started to become self-destructive following her resurrection in Season 6, she had to convince her friends that she was still capable of her duties.
There were times when she abandoned the Scoobies to fight their own battles, and battles that they barred her from participating in. The gang was in a rocky place in Season 7, when they needed to be the most cohesive. Luckily when the world — and her friends — needed her most, she was there for everyone.
Darla
One of the most notorious vampires in the Buffyverse, Darla may not have been featured as prominently as Angel or Spike, but she was capable of committing the same atrocities. And unlike them, she didn't regret her past malfeasance, but she did understand self-sacrifice and ultimately redeemed herself through the power of love.
After becoming pregnant with Angel's (human) baby Connor, she experienced what it felt like to be so close to a soul, staking herself in order to give birth, proving that even the most wicked can be capable of the most comion. Darla died many times, many times, but never for something so meaningful.
Angel
After spending hundreds of years rampaging across Europe to form a reputation for being one of the most formidable vampires on Earth, Angel had to learn to build a different identity, one steeped in guilt, determination, and redemption. His arc was never linear, especially given how he became Angelus again so quickly after being intimate with Buffy for the first time.
Though he often stumbled — torturing Buffy and the Scooby Gang, getting back together with his sire, agreeing to work at Wolfram and Hart to save his child — he always got back up again and continued being a champion for good. His redemption was not only omnipresent and a core part of his character, it was organic and realistic.
Spike
With one of the longest and most progressive redemption arcs in the series, Spike went from being a villain who wanted to watch Sunnydale burn to being a hero that burned himself to save it. He was a vampire who always had a little too much of the poet in him, and a deeper regard for humanity and its failings than he let on.
All William the Bloody ever wanted was acceptance, and through fighting alongside the Scoobies he could achieve that, while through being with Buffy he could find love. Like Faith and Angel, he made his own destiny, becoming a better person with each selfless act of valor.