Basic Trope: After fighting his way to the final villain, the hero lets the villain go to avoid the guilt of killing him.
Straight: After Keith, the villain, killed several people, Bob, the hero, fights his way to Keith. When Bob has Keith at his mercy, he lets Keith go, after Alice, his girlfriend, begs Bob not to "sink to Keith's level".
Exaggerated: Keith has personally tortured, enslaved, and massacred millions of innocent people, and Bob lets him go after Keith merely says "I'm sorry, I can change."
Keith killed his victims as an act of revenge against his victims, who also killed as another act of revenge. If Bob were to kill Keith, the Cycle of Revenge would continue, and Bob would be yet another link in the bloody chain.
Keith needs to do good things every once and while to keep up the good publicity for his Evil Plan to work. But he is worried or warned that he might become more like Bob and thus ultimately decides against it.
Alice begs Bob not to sink to Keith's level... By torturing and killing him.
Alice tells Bob not to kill him, even though he could never stoop as low as him. This guy has feelings and it would be bad, though not as bad as what he had done.
*Smiling, holding several Noodle Implements* "Oh, I won't kill him. There are better things to do to him..."
Alice corrects herself: Torturing or killing.
When he refuses to hurt him, she says that she is proud of him, and that she didn't want to frighten him, but that he would have gotten just as bad as him.
A moment after Alice begs Bob not to sink to Keith's level, she found that Keith's attack also leaves a scratch on her brand new car. And she tells Bob "Just kill him, NOW."
Alice: "Bob, don't kill him. Don't sink to his level! I don't think you can live with his blood on your hands!" Bob: "Okay... hey! Wait a minute! I just butchered my way through 500 of his minions to get to him! What's one more life to me now? Keith: "Bob, forgive me! I know nothing I say or do could give you your dead sister back, but I am truly, truly sorry." Bob: "OK!" (turns to leave) Keith: "Fool..." (Offhand Backhand) "*Hurk... bleagh*" Bob: "Oopsie... Darned reflexes."(smug look)
Averted: Bob kills Keith without flinching.
Enforced: The show has a young audience, and the executives don't want to convey the message that killing solves problems.
Lampshaded: "Are you really comparing me to Keith after what he's done?"
Invoked: "What will make you any better than me if you kill me? We're Not So Different, when you think about it."
" I have to kill him, it's the only way to be sure he never does it again."
Discussed: "If you have a chance to kill Keith, would it be right?"
Conversed: "Wait, you're going to fight your way to Keith and then just let him go?"
Keith continues his reign of terror, and when Bob and Alice return home, instead of being welcomed as heroes, they are demonized, condemned, spat upon, called disgusting words, and later are tried and executed for dereliction of duty and extreme negligence.
Bob let Keith go. Unfortunately, Keith was beyond redemption and the only way to prevent him from doing evil again was killing him. So he continues wreaking havoc upon the town & some people blame Bob because he didn't kill him when he had the chance even though he just wanted to avoid being evil in his-own eyes.