I'm starting to wonder whether the biggest challenge stories about characters who are ideologically opposed to killing face is the resistance their authors have to showing how ugly that sort of violence inevitably would be.
A:tLA and LoK make for a great comparison in this regard. A:tLA wants its audience to agree with Aang that killing is wrong, but when characters do die, it's as clean as possible -- Yue collapses (and returns as a spirit immediately after), Zhao gets dragged down into water by a spirit (and isn't actually dead anyway), Chin fell off a cliff (in a flashback), Professor Zei went down with the library (and wasn't confirmed dead until LoK), Jet died in such an ambiguous way that it became a joke, Aang got hit by lightning only partially on-screen (and came back), Roku got covered by lava, Combustion Man got blown up off-screen, and Kya's actual death was skipped in all three flashbacks.
LoK reverses that trend (to the point of showing the ugly aftermath of Professor Zei's refusal to leave the library!), but then proceeds to treat the much uglier deaths that do happen both as largely necessary and as necessarily ugly. Amon's bloodbending and Zaheer's suffocation and poison have effects that are seriously unpleasant to watch, but there's no sense that they've got cleaner alternatives, since Mako's lightning looks nearly as bad. Even something as clean as Korra's spiritbending is shown to have consequences for the family of the guy she killed (including Korra herself), though it's still positioned as the only viable solution.
But I don't think this weird and kind of counterproductive dynamic is limited to the Avatar franchise. What really made me think of it was Superman vs. The Elite... which, ironically enough, I watched because I figured that if anyone could get me onboard with a no-killing policy, it'd be Superman. (I'm one of the sort who feels like ignoring that part of him kind of misses the point of the character. >.> )
Anyway, Superman had a line near the end where he said that, because of his actions, the people watching "saw the ugliness of violence as a solution and it frightened them," which would have been all well and good if the movie hadn't insisted on using its PG-13 rating for everything but showing lethal violence as ugly.
Seriously, though, it was weird. One villain swore like a British sailor, another was basically there to make suggestive comments, and another drank wine from a bottle in basically every scene he appeared in... but there still appeared to be some sort of policy against deaths being both painful and on-screen. One villain had a power that instantaneously turned people to ash; his head got blown up by an anti-hero, which would have been way less kid-friendly if he hadn't been basically a skeleton in armor. And when there were painful off-screen deaths, they came off as designed to show the anti-heroes as unnecessarily cruel.
And, as a result, what's actually shown is less "lethal violence is ugly" and more "lethal violence sounds painful when committed by sadistic jerks." It sort of undercuts the point.
Since it's pretty clear that the need to keep to a rating wasn't the problem there, though, it made me wonder whether it might have more to do with the mindset of the people going for the "killing is the wrong solution" theme than the limitations under which they work. I suppose that, for them, "lethal violence is ugly" is obvious to the point that it doesn't need to be shown. But for someone like me, whose general philosophy is that justifiable self defense can include lethal violence where no other choice exists, opting out of showing why a hero might see it as too unpleasant to accept as a solution ends up feeling counterproductive. =/