Needs a Better Title
I want to put the conversation in Marvel Civil War about Spider Man indirectly being responsible for the Green Goblin's murders due to not killing him here, but don't have access to it at the moment. If someone could produce that quote, it'd be great.
Superheros (excluding the antiheroic ones) and other stereotypical good guys frequently refuse to kill villains, no matter how horrific their crimes. This trope is what happens when the hero is pretty much the only person who could kill the bad guy, and his refusal to do so is pretty much the entire reason that the bad guy is still killing people. Basically, the hero is somewhat responsible for those killed by the villain simply because he holds his personal code of ethics above stopping the bad guy for good.
Now, this trope doesn't apply when the villain is spared once or twice and then goes out and starts killing again. This trope comes into play when the villain is spared so many times that the hero should know that killing them is the only way to stop them, yet still refuses to.
Usually caused by a Technical Pacifist
- The Green Goblin goes on a rampage. Spider-Man webs him up, and he's put in prison. A few months later, he breaks out and goes on a homicidal rampage again. Spider-Man webs him up, and he's put in prison. How many times does this have to happen before Spider-Man doing the same thing and expecting different results is the real cause of these people's deaths?
- Batman has the Joker (Or just about any other villain) thrown in Arkham Asylum. Eventually, the Joker breaks out and starts wreaking havoc again. Batman has him thrown in Arkham Asylum again. Once again, doing the same thing and expecting different results.
- In Star Wars, Obi-Wan Kenobi leaves Anakin Skywalker lying on the ground, missing multiple limbs. Apparently totally unfamiliar with the fact that technology exists to keep people who have been mortally wounded alive, despite having just fought General Grevious. Admittedly, Anakin hadn't been redeemed and then turned to the Dark Side again, but you'd think he'd figure out that leaving the guy who fell to the dark side over the course of about 24 hours and slaughtered the entire Jedi Order alive might not be the greatest idea. Quite a few bad things could've been prevented if Darth Vader had never existed.
- Revenge Of The Sith has Anakin telling Mace Windu that "it's not the jedi way" to kill Palpatine. So, when Windu proceeds to try to kill Palpatine anyway, Anakin chops off Windu's hand to prevent this, then Palpatine proceeds to kill Windu with force lightning; and that's not even getting started on the rest of the evil Palpatine goes on to do in the series, which could have been prevented had Palpatine been killed. Of course, it's more of an "indirect mass murder by stopping someone else from committing murder against someone who was too dangerous to be left alive," and even then it might be a subversion given the role Anakin's self-interest played in his decision.
- Multiple times during the second season of Buffy the Vampire Slayer, Buffy has the opportunity to kill Angelus, but can't bring herself to do it as she still loves him. That's great and all, but a lot of people would still be alive if she'd killed him.
- Again with Warren, Jonathon, and Andrew in the later seasons. Buffy realistically could've killed them, and a certain point it should've become clear to her that they weren't going to stop doing evil crap. If she'd actually offed them, Tara would still be alive, meaning Willow wouldn't have gone evil and tried to destroy the world. Andrew wouldn't have been alive to kill Jonathon, thus awaking the first evil, meaning that Xander would still have both his eyes and quite a few of the Potential Slayers wouldn't have died fighting off the Turok Han. Basically, a lot of bad things would've been prevented if she'd just killed them after the fourth or fifth time they did something evil.
- This is a common argument for giving the death penalty for certain crimes.