In Astro City, the Street Angel, a Batman-like vigilante who had recently become Darker and Edgier but who still refused to kill, receives a nice bit of smack-talk from murderous antiheroine Black Velvet. She notes that, although he still claims that he never kills, he leaves an awful lot of people with severe internal injuries without actually checking to see whether they survive or receive medical attention. (After she says this, a Beat Panel follows as this sinks in...)
In the WildStorm imprint, in one issue of The Authority, it was mentioned that an alternate earth was essentially destroyed when the Hero refused to kill their enemies no matter what, and the villains killed every single one of them. They attacked the Authority's Earth, and were quickly killed, much to their surprise, saying "Superheroes don't kill". Unfortunately for them, The Authority did. This universe in general, and The Authority especially, fall on the cynical side.
Used for great dramatic effect in ElfQuest. The main tribe of the story, the Wolfriders, have one simple rule: elves don't kill elves. It's a concept so ingrained in their culture, killing others of their kind would not even occur to them. Until one elf from a different tribe, Kureel from the Gliders, ends up kidnapping a young Wolfrider and threatens to kill him. The boy's father (the tribe's archer) shoots and kills Kureel. He goes into a complete Heroic BSoD until he's finally able to ask Kureel's spirit for forgiveness many months later. As it was, since Kureel's spirit was at peace and barely remembered the circumstances of his demise anyway, he granted that forgiveness without hesitation.
Interestingly, at the very start of the series, the Wolfriders seem to treat death much more casually, briefly considering killing Rayek because he looks at them funny. This is, however, shortly after the humans burnt down their home and the trolls betrayed them and left them to die in the desert, and while they were still figuring out what to make of these strange new elves who walked around in broad daylight, had huts, and actually cooked their meat just like their old human enemies did. (It's telling that the Wolfriders decided to 'introduce' themselves to the Sun Village by raiding it for food rather than just walking up and saying hello. Thankfully for both sides, that didn't last long.)
Also interestingly, in the very early days of the Wolfrider tribe, there were many elves born with wolf-blood and just as many wolves born with elf-blood (it's not as icky as it sounds — the first elves were shapeshifters lost on a low-magic planet. Mating with the local fauna was their way of bonding with the new land). Timmorn, the first Chief and first mix between elf and wolf, took on the task of deciding what was elf, what was wolf, and what should just be killed instantly.
Two-Spear didn't have too many qualms either about killing his own daughter. But (a) Two-Spear tried to be more wolf than elf, using the pack's way of life as an excuse to act violently insane, and (b) the story in which he thinks he's killed his daughter was a case of Running the Asylum anyway.
In Empowered, this is played utterly straight with the title character; even her most powerful energy blasts have never been seen to do worse than knock someone out cold. The rest of the cast (including, from the look of things, her costume) averts it, especially Thugboy. In volume 6, she does leave Deathmonger to be disintegrated by a nuclear blast... but he's not only an enslaver of the walking dead, but a walking dead man himself.
Averted in Volume 9 when she kills Fleshmaster.
In Antarctic Press' Gold Digger, the giant superheroine Crush is adamant about this — mainly because, during a brief period during which she was being blackmailed by a supervillain, she killed a bunch of gang members... and, coincidentally, an undercover cop.
Lampshaded in Invincible. After the JLA-analogue repels the same Alien Invasion for a second time (by destroying the devices that allow them to safely exist in our universe), Invincible, as the Naïve Newcomer, wonders at the wisdom of just letting them go again:
Robot: Keeping them here would be a death sentence. Hopefully they've learned their lesson.
Invincible: Right... and I'm supposed to be the new guy.
Robot: It is not mathematically inconceivable that at some point we encounter an adversary that realizes the error of their ways and gives up their plans for revenge.
Invincible: I hope you're right.
Zig-zagged in Judge Dredd. While the titular character has the legal authority to act as Judge, Jury, and Executioner, most crimes don't carry the death penalty in Mega-City One, with the exception of crimes such as treason, murder, rape, or terrorism, so Dredd usually just sentences criminals to terms in the isocubes. However, if a perp resists violently or tries to kill Dredd, he will respond with lethal force, which isn't that far off from how Real Life police officers are allowed to use lethal force when necessary.
Kick-Ass: No way. I'm not going to kill anybody. I'm supposed to be a fucking superhero.
Hit-Girl: Oh, kiss my ass. What is this, the Silver Age? I'm afraid we forgot our magic fucking hypno-ring that turns bad guys into good guys.
In Marshal Law, the Private Eye (a villainous Batman expy) says he will "maim, mutilate, electrocute, gas, or burn" his enemies, but feels self-righteous about doing so because he doesn't kill them.
Played straight in Mighty Morphin' Power Rangers: Shattered Grid. In the second to last part, Rita Repulsa of the "Prime" universe uses the Green Candle on Lord Drakkon and prepares to kill him as he is helpless and losing his powers. Before she can do so, Zordon and Anubis Cruger tell her to stop, that killing him wasn't part of their deal. While she's telling them off for it, it allows Finster-5 to sneak in and take them out.
Subverted in Prince of Persia: The Graphic Novel. Guiv subdues a lion which attacks him in the mountains. Kneeling over the lion, he lifts his sword, then has a thought and says, "No one gave me the power to take life." As he walks away, the lion gets up and charges at him, and he quickly turns and slashes it to death.
Turul: No one gave you the power to spare life, yaaahr.
Enforced in Quantum and Woody by Quantum's heroic idealism. This proves problematic when the duo attacks Terrence Magnum's private mercenary army and Quantum has loaded Woody's rifle with rubber bullets.
In Rat-Man superheroes have the rule they don't kill or even use guns (Rat-Man gunning down a Shadow minion was treated as evidence he was slipping away from being an actual superhero, even when the creature wasn't really alive to begin with). In the Grand Finale the Big BadTopin even mocked him about it, stating that if Rat-Man didn't finish off he'll come back and kidnap his daughter Thea again and again until he can brainwash her into being the next host of the Shadow, but if he does he'll stop being a superhero. Before Rat-Man can decide either way, however, the presumed dead Janus Valkercomes out and stomps on his head.
The Sonic the Hedgehog (Archie Comics) comic has something like this: in one issue Dr. Eggman, still insane after the events of Issue #200, is locked up in New Mobotropolis. A character asks Sonic why he's showing mercy to Eggman. Sonic admits he doesn't know for sure, and guesses he moves too fast to get hung up on revenge. The character isn't sure if Sonic has a Zen state of mind or is foolish, but he's impressed either way.
In issue #225, Eggman mocks Sally for showing mercy on all the times she could've finished him, as doing so allowed him to stay a threat. A few pages later, he seemingly killed her, then reset the Universe.
In Super Crooks, Johnny has a no-kill policy that he enforces on everyone else, though mainly for pragmatic reasons as he doesn't want to be labelled a murderer and making them a bigger target. He has no problem with violent beatdowns that leaves their enemies heavily scarred and/or brain-damaged, however.
The Teenage Mutant Ninja Turtles, being ninja, were pretty much trained to kill from their first appearance in order to avenge Splinter's owner via his death from The Shredder. It varies from incarnation to incarnation with some versions vowing never to kill, but for the ones that do it's made apparent that none of them really relish having to kill anyone period, but if it comes down to survival or saving someone they will take that step.
