). That argument went like this: Killing all evil people would do more harm to society than it would provide in benefits. That's not a reductio ad absurdum. It's straightforward application of the idea behind "the greatest good for the greatest number."
Actually, yes, it's exactly the same thing as an ethical reductio ad absurdum: doing "more harm to society than it would provide in benefits" is the ethical absurdity that prevents us from killing everyone who is evil.
It is not straightforward, because it is argued indirectly, which is all that a reductio is. Straightforward would have been to begin from an initial premise, not its converse. In order to show that a society must not kill all of its evil-doers, you looked at the consequences of what would happen if it did.
From a natural law perspective, I think it could be framed as saying that the universe is crafted in such a manner as to reward good and that evil bears natural punishments. Since killing all evil people would appear to naturally preclude any functional society, that would
That would what?
I don't know where you meant to go with this, but 'would appear' doesn't mean "does" and if your premise doesn't cover all possible societies, then, well, it doesn't.
Traditionally, societies have recognized that killing everyone they recognized as evil would not be right.
Wrong. Traditionally, societies that have practiced capital punsihment quite clearly believe that killing everyone that they recognised as evil would be right. It is perhaps true that they recognised that, practically, they can't accomplish this task, though I would argue that this is more a matter of an acknowledgement of the fact that some evil people get away with their crimes, than that they believed they would kill off too much of their citizenry.
I can't think of a single instance in all of history when a society pardoned a guy of his justly deserved death sentence, because a certain quota of righteous executions had already been met. Can you?
the vast majority of moral codes acknowledged by humans throughout history hold that it is possible to be evil without deserving to die on this earth.
Really? Could you give me a statistical breakdown of that one? Or, tell you what, all you have to do is give me a number: just tell me how many moral codes there have actually been in history (even better: human or otherwise!). Yeah; you go ahead and provide me with a farily credible list of every moral code that has ever existed, and I won't even argue the point: I'll just assume that you've examined each and every one of them at enough depth to assess their views on capital punishment. Do me a favour, though: start with New Guinea.
10-20% of the population is quite sufficient to render irradication a non-option.
If by this you mean "eradication by radiation," you may be right. If you mean, instead, "mass execution," you're wrong: it's entirely possible for a society to survive the death of 10-20% of its population. Furthermore, if all evil or even most evils can be traced infallibly to its or their source--evil people--well, yeah, it's perfectly reasonable to expect a society to go ahead and off them.
But this accepts the point that even 10% of a population is necessarily evil. And that's bologna.
If you wish to defend the idea that any human society could ever possibly be even be as universally free of evil as the Western world is of illiteracy, you need some argument for it. (Every single fact of human history points in the other direction).
First, I wouldn't make this comparison, because the Western world is not universally literate (given how much of an issue illiteracy is just in the States, what are you even talking about?). Second, I live in a fantasy world, ok? I don't need to justify the functioning of a magic spell in my make-believe society with the moral facts of the real world. Nor is it necessary for me to do so, even if I want to adhere like the craziest of pedants to the core rules of the Dungeons and Dragons role-playing game.
The unworkability of eliminating all evil is especially unworkable because, unlike an extermination program against a race which can be accomplished once and for all, the anti-evil pogrom would have to be ongoing in order to root evil out of both the new generations and those of the old generation who have fallen into evil since the last mass-killing.
Only if I decide by fiat that a certain percentage of the new generation is born evil, and, even if I do this (and I won't), I can quite infallibly eliminate them, too. As for those of the old generation, well, they wouldn't necessarily go evil, unless we assume that a certain portion of the population must be evil and that they've changed teams in order to fill this role. What a circle! Of logic!
No society can sustain such losses on a continuing basis.
Sure they can. They can increase their birth rate.
And the idea that, just because killing them would be the right thing to do doesn't mean it will happen is absurd as an ethical defense of anything. As Kant pointed out, ethics have to be universal. Anything else is ultimately unworkable.
Actually, Kant pointed out that ethics are not universal, that they cannot ever be so, and that we should, all the same, pretend that they are.
I don't know what "an ethical defense of anything" is, so I can't speak as to its absurdity level, but the point I made was that ethical beliefs don't necessarily lead to practical consequences. It was the practical consequences of the belief that all evil persons should be killed that you were complaining about.