"It’s your one last chance ...it’s all you’ve got left. . . . I’ve seen what you’ll be otherwise. . . . Be a man . . . try . . . Try for some remorse. . . .”
-- Harry to Voldemort at the end (DH36)
A deep sense of guilt, sorrow and regret over an act that causes harm to another person.
References from the canon
Remorse has the magical power to heal and repair the torn souls of those who cause death and pain to others. Remorse is connected to love and empathy for other people, and the ability to humbly confess mistakes and past sins.
In the book Secrets of the Darkest Art concerning how to make a Horcruxes, Hermione read that the process of remorse takes years of sacrifice and require suffering that could lead to death; As she simply put it "You've really got to feel it" (DH6).
Voldemort lacked a conscience and the ability to care for anyone but himself. Frank Bryce was chilled when he overheard Voldemort in the Riddle House talking about killing Bertha Jorkins "without any kind of remorse - with amusement" (GF1). Before their final duel, Harry gave Voldemort multiple opportunities to "try for some remorse," appealing to his human side as "Tom," the same way Dumbledore once did (DH36). While Voldemort was shocked that Harry spoke to him in such a forceful way, the Dark Lord seemed to have no interest in repairing what little soul he had left. However if Voldemort had shown any level of remorse, he could have been "healed more deeply than anyone would suppose" (JKR).
Characters who admitted wrongdoing and expressed remorse:
Albus Dumbledore spent much of his life in remorse for the way he treated his mother and his younger sister Arianna, and he felt responsible for both of their deaths. Arianna had been attacked by Muggle children, and as a result of the trauma could not control her magical powers. His father was locked up for taking revenge on the boys who attacked her, while his mother, Kendra, was killed accidentally when Arianna's magic went haywire. Dumbledore was young and arrogant and bitterly resented having to return to Godric's Hollow to care for his disabled sister as well as his younger brother Aberforth. As Dumbledore told Harry, "I returned to my village in anger and bitterness. Trapped and wasted, I thought!" (DH35). When Grindelwald arrived in Godric's Hollow things became worse as his ideas "inflamed" Albus with the idea of making Muggles subservient to Wizards, as well as hunting down the Deathly Hallows to help them do it. They hoped to become "Invincible masters of death, Grindelwald and Dumbledore!" (DH35).
All those plans ended when Albus, Aberforth, and Gellert ending up in an argument that turned into a magical fight in the same room as Arianna, who ended up dead, struck by a curse. Dumbledore felt such grief, sorrow and remorse that he retreated from public life to Hogwarts, where he tried to make up for his past mistakes by training the next generations of wizards. Eventually he personally defeated Grindelwald, taking from him the Elder Wand in a duel and ending World War II, and instead of killing him made sure he was locked up for life. All the remorse led to Dumbledore's own death, as he put on the Peverell Ring containing the Resurrection Stone because he wanted to see the spirits of his mother and sister again and tell them how sorry he was. The curse on the ring gave him only one year to live, leading up to his death at the top of the Astronomy Tower.(DH35)
But there other sources of remorse in Dumbledore's life, much of which he confessed to Harry - guilt about bringing Tom Riddle into Hogwarts and not realizing his capacity for evil, guilt over the deaths of James and Lily Potter after taking James's Invisibility Cloak still in hopes of owning all three Hallows, guilt over the deaths of Cedric Diggory and Sirius Black, guilt over what Harry had to endure as a child at the Dursleys, and even remorse in the afterlife for the death of Severus Snape. Considering all the remorse Dumbledore felt at all times, there is little doubt that his soul was in a constant state of repair.
Gellert Grindelwald, once known as a "Demon" for his evil deeds during World War II, was defeated in a duel with his old friend Albus Dumbledore, who took the power of the Elder Wand from his grasp (PS6). When Voldemort arrived at Nurmengard Prison asking about the Death Stick, Grindelwald lied and told him he had never owned it. Dumbledore told Harry he believed the lie Grindelwald told was a sign of remorse, that he wanted to "make amends ... to prevent Voldemort from taking the Hallow" (DH35). Harry wondered if Grindelwald was thinking of Dumbledore at the end, and did not want Voldemort to break into his tomb out of love or respect. But Dumbledore hoped it was true that “he showed remorse in later years, alone in his cell at Nurmengard. I hope that it is true. I would like to think he did feel the horror and shame of what he had done" (DH35) because that would mean that Grindelwald had repaired his torn soul.
Sirius Black felt guilty over the deaths of his friends James and Lily because the plan he formed to make Peter Pettigrew the Secret Keeper under the Fidelius Curse was what led to their deaths: "Harry... I as good as killed them," he croaked. "I persuaded Lily and James to change to Peter at the last moment, persuaded them to use him as Secret-Keeper instead of me.... I'm to blame, I know it...." (PA18). When he realized that Peter was still alive and running around free as a rat, he broke out of Azkaban to confront Peter and avenge James and Lily's deaths, but Harry stopped Lupin and Sirius from killing Peter. Sirius never quite felt remorse for his bullying of Severus Snape as a boy, but he did admit to Harry "I'm not proud of it" (OP29).
Peter Pettigrew never came right out and expressed remorse for his role in the deaths of James and Lily, telling Sirius and Remus that he had no choice but to go along with Voldemort out of fear when he sold them out (PA18). But after Harry spared his life in the Shrieking Shack, he tried to persuade Voldemort to use anyone except Harry in his plan to be reborn: "The boy is nothing to me, nothing at all! It is merely that if we were to use another witch or wizard -- any wizard -- the thing could be done so much more quickly!" (GF1). The Dark Lord knew Peter was lying and sensed his regret, which is probably why Voldemort repaid the sacrifice of his hand for his rebirth potion with a silver hand that would kill him for disloyalty (GF32, GF33). When Harry appealed to Peter's remorse at Malfoy Manor he thought he saw mercy in his eyes just before the unforgiving hand strangled him.
Igor Karkaroff told the Wizengamot that he had denounced Voldemort and "was filled with remorse so deep" he could scarcely breathe while talking about his fellow Death Eaters (GF30). Rather than go back to serve the Dark Lord, Karkaroff fled and was later killed (HBP6).
Regulus Black, brother of Sirius Black, became a Death Eater soon after leaving Hogwarts. At the time he was completely enamored of the Dark Lord, filling his Slytherin-green room at Grimmauld Place with news clippings about Voldemort's rise to power (DH9). He was also the "good son" of the Black Family, trying to uphold their beliefs in blood purity and Wizarding superiority (OP6). But all of that changed when Lord Voldemort took the Black family's House-elf, Kreacher, to visit a sea cave where he was forced to drink the poisonous Cave Potion to test the protection on one of the Dark Lord's Horcruxes. Voldemort threw Kreacher away and left him to die, but thanks to House-elf magic, he bypassed the Inferi in the water and apparated home, where a bewildered Regulus heard his story. His affection for Kreacher filled him with anger at the Dark Lord, and when he realized that the hidden object in the cave was a Horcrux that would make Voldemort immortal, Regulus decided to destroy it (DH10). His brother Sirius knew that Regulus tried and failed to leave the Death Eaters (OP6), but he had no idea that remorse over his mistake had led him to sacrifice his own life: Regulus returned to the cave with Kreacher, but this time he drank the evil potion and was pulled under the water by Inferi. Kreacher returned home to mourn his passing and never forget him (DH10).
Horace Slughorn was Head of Slytherin House back when Tom Riddle was a star pupil at Hogwarts. In fact he was so looking forward to the bright future of the popular young Tom and all the gifts he might receive from him that the Professor Slughorn overlooked what a devious psychopath he was. When Tom asked him how to create a Horcrux, a device containing a piece of soul torn off after a murder, Horace was shocked but explained it to him in spite of his misgivings. The memory of it caused him such deep shame even years later that he tampered with his own memory to make it seem as if he had never answered Tom's question. Adding to his remorse was the fact that Tom became Lord Voldemort and terrorized the Wizarding World, ending with the death of Lily Evans Potter, one of his favorite students and Harry's mother. During Harry's sixth year, Slughorn came back to Hogwarts to teach Potions, and Dumbledore used Harry to appeal to Horace's guilt over the death of Lily and James to persuade him to supply the real memory of the Horcrux conversation (HBP4, HBP22)
Severus Snape was a young Death Eater when he overheard a partial prophecy from Sybill Trelawney at the Hog's Head Tavern and ran with it back to his Master, Lord Voldemort. His life of remorse began soon after when he realized with a shock that the Dark Lord believed the prophecy concerned the unborn child of his lost love, Lily Evans Potter. Snape begged for her life to be spared and Voldemort agreed, but Severus was so desperate to save Lily that he defied the Dark Lord and met with Dumbledore on the Windy Hilltop, offering to do "anything" to save Lily's life. Dumbledore told Harry: "'You have no idea of the remorse Professor Snape felt when he realized how Lord Voldemort had interpreted the prophecy, Harry. I believe it to be the greatest regret of his life and the reason that he returned -" (HBP25). When Lily died anyway, betrayed by Peter Pettigrew, Snape was devastated and wanted to die, but Dumbledore encouraged him to live and protect her son Harry in case Voldemort returned one day.
“Her son lives. He has her eyes, precisely her eyes. You remember the shape and color of Lily Evans’s eyes, I am sure?” “DON’T!” bellowed Snape. “Gone. . . dead. . . ” “Is this remorse, Severus?” “I wish. . . I wish I were dead. . . ” “And what use would that be to anyone?”
“If you loved Lily Evans, if you truly loved her, then your way forward is clear.” Snape seemed to peer through a haze of pain, and Dumbledore’s words appeared to take a long time to reach him. “What—what do you mean?” “You know how and why she died. Make sure it was not in vain. Help me protect Lily’s son.” (DH33)
And Snape did just that for the next sixteen years, following orders from Dumbledore and attempting to repair his soul while becoming a double spy for the Order of the Phoenix. Snape used his remorse as a smokescreen to Bellatrix Lestrange, explaining that he lied to Dumbledore: "I spun him a tale of deepest remorse when I joined his staff, fresh from my Death Eater days, and he embraced me with open arms" (HBP2), a story which was actually true, although Bellatrix had trouble believing it. Snape used Occlumency to hide his deepest feelings and true motivation from both Bellatrix and the Dark Lord successfully. But after saving Dumbledore's life from the cursed Peverell Horcrux ring and giving him one more year to live, Snape was told he then had to kill the Headmaster in order to spare damage to Draco Malfoy's soul, which caused Snape to wonder if Dumbledore cared about the state of his own soul:
“That boy’s soul is not yet so damaged,” said Dumbledore. “I would not have it ripped apart on my account.” “And my soul, Dumbledore? Mine?” “You alone know whether it will harm your soul to help an old man avoid pain and humiliation,” said Dumbledore...(DH33)
Thus in order to protect Draco's intact soul, Snape had to sacrifice and possibly tear his own soul for the second time, perhaps the ultimate act of remorse even greater than giving his life while trying to defeat the Dark Lord (HBP27).