That interesting formulation - focused on the morality of the execution rather than on the vengeance - stuck in my mind. So I'm reading Nettle & Bone by T. Kingfisher, which is the topic for our book discussion group this week. It includes a princess from a small kingdom who's trapped in an abusive marriage with the cruel prince of a much larger kingdom. She can't leave him because he'd take it out militarily on her homeland if she did; the marriage was purely for diplomatic purposes anyway.
So in this scene, the princess's sister is discussing the problem with a hedgerow witch-type and declares, "He deserves to die." The other replies, "Lots of people deserve to die. Not everybody deserves to be a killer."
How interesting: almost exactly the same formulation. (The opera came first.) I've even seen this argument given primary-world application, in the form of articles discussing the moral toll that being an executioner, or even a participant or witness, in death-penalty states takes on those who do it.
Of course the conversation in Nettle & Bone also reminded me of this, and I hope I don't have to tell you where it's from, who's speaking, or who they're talking about:
"I can't understand you. Do you mean to say that you, and the Elves, have let him live on after all those horrible deeds? Now at any rate he is as bad as an Orc, and just an enemy. He deserves death."
"Deserves it! I daresay he does. Many that live deserve death. And some that die deserve life. Can you give it to them? Then do not be too eager to deal out death in judgement. For even the very wise cannot see all ends."