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These guys are guards for evil priests who capture innocent people for bloody sacrifices & are buddy buddy with all sorts of vile monsters whose alignment entries in the MM read "always evil." No neutral or good guys would hang with these cultists and claim "just doin' my job" for more than about a day before finding their alignment shifting to evil. I also view the guards as being members of the cult -- they are not priests, but I expect they take part in the ceremonies and rites ... My players have no compunction about executing these bas*ards after they have questioned them -- and frankly I as DM would be more likely to look at alignment if they let one of these "monsters" go free after questioning.
Good characters are bound to accept surrender and treat prisoners properly. Torture is out. Summary execution is out. Cutting down a surrendering foe is out. Those are evil acts. That said, a good character doesn't need to be stupid about prisoners. Bind, gag, and blindfold them, just don't torture them.
The Players Handbook says "Good" protects innocent life. Good does things for others. Says nothing about the killing of evil people. Therefore, if players have a helpless, helpful prisoner who is bad, it is fine to kill him after interrogation. It is fine to inflict pain to get that interrogation. No evil in it. The worst dilemma is how to get the blood off your armor.
If you inflict pain on the prisoner after you got what you want out of him? That's evil. If you interrogate and kill a shopkeeper who doesn't want to tell you some piece of info? That's evil.
Good characters and creatures protect innocent life ... "Good" implies altruism, respect for life, and a concern for the dignity of sentient beings.
Is a coup de grace attack an evil act? (i.e., can a paladin make such an attack without falling from grace?
The coup de grace is simply a kind of attack and is neither inherently good nor inherently evil. In some cases, it's the best option against a foe (such as an unconscious but regenerating troll). If attacking a particular character would be considered an evil attack -- such as stabbing an innocent merchant in the middle of his shop -- delivering a coup de grace would be just as evil.
Of course, if the paladin has already promised to face her foe in nonlethal combat, delivering a coup de grace would almost certainly violate her code of conduct.
Prisoners must be treated with a certain amount of respect. Torturing prisoners is out of the question, of course, and generally knocking prisoners unconscious every time they wake up amounts to cruelty ... There is one good and important reason to take prisoners rather than kill every enemy: live prisoners give better information than dead ones ... While evil characters readily resort to torture to extract information from prisoners, good characters simply can't, and even using the threat of torture ... is morally questionable.
Good characters and creatures protect innocent life ... "Good" implies altruism, respect for life, and a concern for the dignity of sentient beings.