Go and Sin No More

The villain has either been defeated or acknowledges the error of his ways. He surrenders to the hero, expecting to be killed or punished in some other way. Instead, the hero lets him go, telling him to repent for his crimes by doing good deeds. This usually turns the villain into The Atoner and send him on a Redemption Quest.

Trope Namer is The Bible, in which Jesus releases an adulteress rather than stone her to death, as was common at the time.

Anime and Manga

 * Done several times in Rurouni Kenshin.
 * Goku of Dragonball Z tries this on Friesa twice.
 * This happens in the Nuts Fujimori manga adaptation of Fire Emblem: Seisen no Keifu. In this adaptation, one of the early bad guys, Kinbois of Verdane, is portayed as deep down not being evil, just someone influencable who was following the wrong path. Sigurd notices this, and lets him live on the promise he will atone for his sins. Kinbois is genuinely moved by this mercy and promises. This turns out to be crucial, as Kinbois' brother Jamuka later fights Sigurd because he believes, in a misunderstanding, that Sigurd has killed his beloved brother in battle: after learning the truth and seeing Sigurd's generosity, Jamuka joins his party.

Film

 * In Gandhi a Hindu comes to Gandhi as he's partaking in a fast and says he (the Hindu) is going to Hell, because he killed a Muslim child in revenge for Muslims killing his son. Gandhi tells the man to to repent by finding a Muslim boy whose parents have been killed, and raise him. But here's the kicker: he's to raise the boy as a Muslim.

Mythology and Religion

 * The Trope Namer is a story in The Bible.
 * If you identify "the hero" in the page description with God, you'd have a fairly accurate representation of the Christian doctrines of repentance and salvation.

Literature

 * Done by R. Daneel to the corrupt police chief in Caves of Steel by Isaac Asimov. Instead of arresting the chief for the murder of a pro-robot, pro-space travel scientist, Elijah and Daneel tell him to rally support for robots and space travel among the Luddite "Medievalists" he worked with.
 * Subverted in Wizard's First Rule. Richard forgives for everything he did to him. However, Richard still executes him as he does not forgive him for his sins against others.

Live Action TV

 * In Doctor Who, the Doctor makes a Sontaran work as a nurse.
 * In The Sarah Jane Adventures, an alien who had been kidnapping children to use as child soldiers realizes that the war his people were fighting had ended without his knowledge. He allows Sarah Jane Smith to take his life, but she refuses. The alien then promises to return to his homeworld and bring back the humans he had abducted.

Video Games

 * In several Mass Effect games, you can either kill an enemy as a Renegade decision or let him go for Paragon points. Most of the time it will usually work out in Shepard's favor to spare the character.
 * In the The Legend of Zelda CDI Games, after Duke Onkled betrayed the King of Hyrule by siding with Ganon, he gets off by being told to "Scrub all the floors in Hyrule."