So, because of mechanical alignment, the entire group becomes murder hobos. An enemy can never surrender in your world? Who cares if the party ever takes prisoners after all, we don't need all that pesky role playing stuff. It's better just to kill every opponent outright. Saves on all that unimportant stuff like talking and whatnot. Why bother giving players the choice?
The fact that you have flat out stated that the paladin executing a prisoner is murder, tells me, the player, that I can never, ever execute a prisoner. Which means that I will never take prisoners because it will become nothing but a huge PITA.
You've pretty much precisely outlined exactly why I don't like mechanical alignment.
This seems much less like an alignment issue and much more like a role playing issue. The player wants to be able to kill helpless prisoners on a whim, and does not want anything precluding him from doing so. Like that pesky role playing of someone who actually respects life and considers slitting the prisoners' throats to be a morally repugnant act. I just bwant my character to do whatever is most tactically effective and/or practically expedient at any given time.
Sorry, but, to me, "whatever is best or easiest for me, regardless of the impact on others" is neither heroic nor Good.
Not every wrongdoer is a hypocrite. If you won't except Lenin or Trotsky, what about (to pick someone who is less likely to trigger immediate accusations of hypocrisy) LBJ?
Who said we don't accept that Lenin or Trotsky might sincerely wish the best for the people as a whole? We accept that a Paladin who goes to war against an uncaring monarch is Good, which seems much like a freedom fighter who goes to war against a government which does not act in the interests of the people. Bringing real world history into it colours the issues, undoubtedly. Are you suggesting that other world leaders of the time who vilified the Communist regime out of concern for protecting their own positions of power and privilege also had the best interests of the people at heart?
The thing is, how can you judge whether a political compromise is good or evil - compassionate or not - without taking a view about social order, the permissibility and desirability of sacrifices and tradeoffs, etc - which are ostensibly elements of the orthogonal dimension of Law/Chaos?
"Compromise" implies neither 100% one thing nor 100% the other. The possibility that a choice could be nether good nor evil (or neither lawful nor chaotic), having elements of both, seems to completely escape your framework.
It is comparatively trivial for the person in question to cast the spell him-/herself, eg from a magic item. A king who was worried that s/he was doing the wrong thing would simply test 1x/day with his/her intelligent sword +1.
Does he believe that intelligent sword (which, being intelligent, has motives of its own)? Does it matter? If he has taken matters to the point that his alignment has already changed, he's already done quite a bit of "the wrong thing", hasn't he? He may very well be looking at the overall picture and deciding that he knows what is best for his people, and if the people of a neighbouring land must suffer for the benefit of his people, then that is an acceptable sacrifice. It's well and good for these philosophers or higher beings to look down from their lofty towers, but he is right here in the thick of matters, and must deal with reality, not ethical philosophical theory. And so he can rationalize his actions, just as we can rationalize ours.
Does it matter how sincerely he believes that genocide is the morally right action to pursue, or is the pursuit of genocide in itself evil?