How Dare You Spare My Life

The hero has just fought his enemy--perhaps the Big Bad, perhaps The Dragon--and defeated them without killing them. He may have disarmed them, or he may have a knife to their throat, or they may have been knocked out for the moment. Undoubtedly, lots of people in the audience are crying "Finish Him!! He's a Complete Monster!"

Not so fast. Killing a helpless person would not be heroic. The hero will not stoop so low, or rather, the writer does not want to tarnish the hero's reputation by having him stoop so low. At the same time, however, the writer would kind of like to kill off the villain. So the villain, despite being defeated, launches a desperate attack on the hero. Perhaps the villain is outraged over their Arch Enemy extending them this kind of charity. Perhaps they genuinely think they can kill the hero while his guard is down. Or perhaps it's something altogether different. In any case, this attack will usually fail and give the hero an excuse to kill the no-longer-helpless villain in self-defense. Then the villain is dead, the hero's still on the moral high ground, and everybody's happy.

Comic Books

 * After the Green Goblin killed Gwen Stacy, Spider Man tracked him down and beat him nearly to death. Spidey was so angry that he wanted to kill the Goblin, but at the last minute stopped himself. He thought that Osborn was no longer a threat and Osborn, who was still able to remotely control his goblin glider, positioned it behind Spider-Man and hit the gas hoping to impale him. Spidey dodged the glider and it hit Osborn instead, killing him. At least, that's how the story originally went.
 * Wolverine, in the final issue of the "Kitty Pryde & Wolverine" limited series, had defeated Ogun and brought him to his knees. He spared Ogun's life and began to walk away alongside Kitty. Ogun produced a knife and rushed at the two of them from behind. Wolverine noticed it in time to tell Kitty to use her phasing power, causing the knife to pass harmlessly through her. He then stabbed Ogun with his claws, killing the villain.

Literature

 * Subverted in "The Silent Blade" by R.A. Salvatore. Drizzt Do'Urden and Artemis Entreri have engaged in a duel to the death to determine once and for all who is the better fighter. Entreri ultimately loses, although even Drizzt acknowledges that his loss has more to do with bad luck than any lack of skill on his part. Entreri doesn't care, and tells Drizzt to finish him since he cannot live with the knowledge that he was beaten. Drizzt refuses, and begins to walk away. Entreri runs at him from behind and cries out in rage--his goal, as it turns out, is to alert Drizzt to his attack so that Drizzt will be forced to kill him. And Drizzt does defend himself by turning around and stabbing Entreri. However, a protective spell cast on Entreri without his knowledge protects him and mortally wounds Drizzt instead.