It's funny, this exact issue came up a couple of months ago with a newbie DM during the second game session:
She had three evil foes teleport in and attack our group. We slew two of them, and disabled the third, who we promptly tied up. Then we healed him, interrogated him, and quickly slew him.
The DM ruled that the act was evil. (Regardless of the fact that the guy detected strong evil and attacked our party for no reason.) We all argued that the act was neither good nor evil, but definitely chaotic and neutral. Chaotic because we, not tradition or a government or a code decided his fate, and neutral with respect to good or evil because it fulfills none of the criteria for good or evil out of the PHB (or even a loose interpretation of such.)
(Hence the reason why St. Cuthbert is LN instead of LE.)
She then had a bunch of 15th level arcane archers appear and demand that we surrender to them. When we resisted (we were not within their jurisdiction), these CHAOTIC GOOD elves slaughtered the GOOD party because they were under
orders to do so. (And they knew we were good because they had previously cast detect good.)
We argued that the elves were being LAWFUL EVIL.
Needless to say the campaign ended right there.
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My other group actually talked about this tonight and we managed to come to a consensus that there are six types of actions:
lawful
neutral
chaotic
good
neutral
evil
(Just in case this wasn't obvious, I am talking about DnD morality, not real world morality.)
The PHB implicitly says that there are actions that are neither good nor evil. (Such as eating some rations or taking a leak.) These are morally neutral actions. Defending oneself is a morally neutral action. Interrogating someone is morally neutral. Killing someone after they attempt to take your life is neutral.
Whereas torture is evil. Hacking off someone's limbs, casting cure light wounds to staunch the bleeding, and then leaving their mauled body to slowly die of starvation is evil.
The problem arose when a PC chose to take a morally neutral action into their own hands. In the example above,
our characters chose whether the prisoner deserved to die, not some independent arbiter (or tradition or code or what have you). Our characters were being
chaotic, not evil.
Hence the reason that an executioner tends to be LN. If interigation and execution were evil, then all governments (and the individuals employed within) in DnD would be evil.
Anyway, my opinions (again).