An action can be the correct action to take and still be Evil. For example a group of good aligned characters has taken the Big Bad prisoner, do they take him in to face justice knowing he may well escape, or do they execute him here and now knowing the act of killing a helpless opponent in cold blood is an Evil one?
This is where I have a fundamental problem. As a piece of English, "This action is Evil but is nevertheless morally correct" is contradictory in nearly all cases.
If, in fact, for whatever reason, it is morally permissible to kill the prisoner, then doing so is per se not evil. If it is morally obligatory to do so, then doing so is good. That's what the words "good" and "evil" mean!
To tie this back to the 3.5 SRD definitions:
"Good" implies altruism, respect for life, and a concern for the dignity of sentient beings. Good characters make personal sacrifices to help others.
"Evil" implies hurting, oppressing, and killing others. Some evil creatures simply have no compassion for others and kill without qualms if doing so is convenient. Others actively pursue evil, killing for sport or out of duty to some evil deity or master.
Nothing there tells us, without interpretation and application, that killing the prisoner is evil (it is not motivated by a lack of compassion, nor sheer convenience, nor sport; and D&D has never taken the pacifistic view that all deadly violence is evil as such). To reach that conclusion we have to reach the conclusion that doing so shows a lack of respect for life and a lack of concern for the dignity of sentient beings.
I assume that the reason for killing the prisoner here and now is that the balance of interests in respect of life favour killing him/her now rather than taking him prisoner, and risking escape and therefore the lives of those the escaped prisoner might take.
There are of course well-known arguments against such a process of weighing up the interests in play and deciding what is the right thing to do (see, for instance, any criticism of strictly consequentialist moral theory) - but those argument are arguments that it is
not correct to kill the prisoner, that to do so would be
evil. Rebutting those arguments
is rebutting the claim that to do so is evil. What counts as proper respect for life and dignity is precisely what is at stake in the debate between the consequentialist and his/her opponent.
Alignment has nothing to do with what's "right" unless the GM decides that Good is always right and Evil is always wrong.
This is like saying that 2 x 2 <> 4 unless the GM decides that it is. At a certain point the logic of ordinary usage has to be given priority or the game terminology becomes unworkable. "Good" and "evil" are defined using other evaluative notions like "respect", "dignity", "altruism" etc. You can't just ignore these words and suppose that it is up to the GM to decide whether or not it is a good thing to be good. Of course it's a good thing! It's practically tautologous that this should be so!
A character who's largely Chaotic but has one or two Lawful traits (being honourable, keeping to their word) is still Chaotic, they just aren't at the extreme of Chaotic behaviour.
I am somewhat curious what's left of the largely "Chaotic" character who is an honourable promise-keeper.
Per the 3.5 SRD:
"Law" implies honor, trustworthiness, obedience to authority, and reliability. On the downside, lawfulness can include close-mindedness, reactionary adherence to tradition, judgmentalness, and a lack of adaptability. ]
<snip>
"Chaos" implies freedom, adaptability, and flexibility. On the downside, chaos can include recklessness, resentment toward legitimate authority, arbitrary actions, and irresponsibility.
A person who is honourable, trustworthy and a promise-keeper is also going to be reliable (that's a consequence of being honourable and keeping your word) and also probably obedient to authority (at least once that authority is acknowledged, perhaps by way of promise). Where is the adaptability and flexibility? The irresponsibility?
Whereas Good gives only one unique fundamental principle -- helping others -- Law is a mishmash that includes being honorable, having a code, respecting authority, following tradition, valuing social order, disliking change, lacking creativity, etc.
I personally don't see them as a mish-mash at all. They capture the classic conservative value of respect for tradition. Whereas chaos is (among other things) transformative.
My problems with law and chaos are not that they are hard to pin down, but that (i) they aren't really oppositional at the individual level (as I've said, the monk and the bard can generally get along fine), whereas D&D wants them to be, and (ii) in real-world moral argument they are not orthogonal to good and evil, but rather candidates for being labelled good or evil depending upon broader moral perspective (eg JS Mill, together with the other classical utilitarians, clearly thinks that D&D-style "law" - respecting tradition, valuing social order, disliking change, etc - is a social evil which impedes progress).