This thread prompted me to go back to Roger E Moore's article "It's Not Easy Being Good" (Dragon 51, July 1981).
This is Moore's general account of how paladins should respond to evil:
Once a Paladin is aware that evil exists, he cannot turn his eyes away from it; that’s not just cowardly, it’s wrong. On the other hand, a Paladin cannot just slay every evil person he or she meets. That’s wrong, too, unless every evil person one meets is trying to kill the Paladin or someone else. Somehow, in one way or the other, the ends of evil must be undone.
And he then says this about killing:
Killing is a difficult topic to address with regard to Paladins. . . . In an AD&D game . . . there are many creatures whose whole existence is evil and cannot be undone by any means short of a Wish (and even that may not be possible). Undead of any sort, evil dragon types, and all demons, devils, and daemons deserve (from a Paladin’s point of view) no other fate than utter and absolute destruction. . . . There is no quarter and
no prisoners are taken.
Other beings, like Beholders and Mind Flayers, will also fit pretty well into this category. No amount of polite talk and reasoning will convince an Intellect Devourer to be a nice guy. The sword is the only answer. When orcs, trolls, and so forth are encountered, the same applies. They are evil, there are deities who make a living at keeping them evil, and there’s not much more to say. Perhaps the only exceptions one could make to killing evil monsters would be if they surrendered; the Paladin could then tie them up or whatever and march them off to the nearest authorities to stand trial or be imprisoned.
Obviously none of this is official or authoritative, but it does show how one thoughtful and influential D&D personality saw the issues 40-odd years ago.
Moore doesn't tell us how what he thinks is best to do if (due, say, to a high roll on the reaction dice) a group of orcs approaches a paladin in order to parley. Is this still a case where the sword is the only answer? Or is it a case of an evil person who is
not trying to kill the paladin or someone else and hence who is not liable to be killed? I think there's no obvious answer here, and probably no typical answer either. The Mouth of Sauron clearly is as evil as any orc ("more cruel than any orc" says JRRT), but in Book V of LotR Aragorn did not threaten him (though he did cow him with his presence) and Gandalf spoke with him as with any other herald.
In the Director's Cut of Return of the King, though, Aragorn decapitates the Mouth. Some (eg me) consider this an unwarranted and (at best) distasteful departure from the novel; others (eg Peter Jackson) clearly thought it a reasonable depiction of a just king's conduct.
For anyone who wants to run a "morally unambiguous" D&D game, I would suggest not having orcs seek to parley.