So, you have an enemy or threat completely at your mercy. They've proven they can and want to harm you and/or your interests, and have probably even done so in the past. However, as they beg for mercy they promise they'll give up (and possibly even join you), and mention they have a family that depends on them. So the choices are: Show mercy and take the risk that they're lying and will strike again, or kill them and end up with a very angry teen hero in a few years or months. Sure, there's probably going to be a lot of sub-options and Shades of Grey, but by and large no matter which choice is made it'll likely come back to bite you in the ass. This applies almost equally to heroes and villains. Evil Overlord who embarked on genocide because of a prophecy? Nice Job Breaking It, Herod!, it's a Self-Fulfilling Prophecy. On the other hand, if they take the children of all vanquished enemies as slaves or even as a children? Expect revolt and betrayal as the hate simmers and they discover the truth. Heroes usually get less problems with showing mercy. However, most villains when offered a Last-Second Chance choose to betray the hero immediately and end up as a Self-Disposing Villain, though some are more cautious. If the hero doesn't? Congratulations, if it isn't If You Kill Him, You Will Be Just Like Him then a loved one of the baddie will engage in Avenging the Villain. Much like Forgiveness, Mercy really is a difficult virtue to practice. Compare A.I. is a Crapshoot and Curiosity Is a Crapshoot.
- In Underworld, vampire elder Viktor killed Selene's entire family out of thirst for blood, but couldn't bear to kill her because of her resemblance to his late daughter. So instead he turned her and cared for her as if she really were his daughter, and lied saying that it was a pack of werewolves that did it. Of course, once the truth came out she wasn't very happy.
- In Thor, Odin showed mercy to frost "giant" Loki, adopting him as a son. While his villainy and ultimate goals are debatable, his conflicted nature and loyalties were the major source of conflict in the plot.
Hello, Unknown Troper. You'll need to get known to lend a hand here.