Dr. Awkward said:I'm still waiting for someone to explain why you can't CdG an opponent who falls to a spell, but you can allow opponents downed by violence (now "prisoners," by your logic) to bleed to death until they hit -10 HP.
This is one of those grey areas that's even more complicated in D&D than it is in RL. In RL, a gun is a pretty lethal weapon and the only way to defend yourself against it is with another gun. In D&D, a hero has a little more latitude in how he responds to a threat. Also, in RL, humans enjoy the distinction of being the singular highly-intelligent sentient lifeform on the planet (as far as we can tell), while in D&D sentience comes in different forms and sizes. Is a kobold's life every bit as precious as a human's? How about an otyugh; they possess rudimentary Intelligence, and some can even speak Common.
Situations like this are covered by books like the BoED. Slitting a sleeping kobold's throat could be deemed cruel, but so could blowing him up with a fireball.
Here's my take on it:
If an opponent poses an immediate threat to your life, responding with lethal force is pretty justifiable, morally and logically. In the heat of battle, no quarter may be expected or given. Even if your foe winds up dying on the ground, the mortally-wounding blow was delivered as an act of self-defense.
When people speak of "killing in cold blood", they are referring to the act of taking the life of someone who poses no immediate threat to you. You take that life purely out of a deliberate desire to slay, not because circumstances forced your hand. Cold-blooded murder can be logical, but if your morals happen to incorporate a fundamental respect for life--as the morals of good people tend to--then killing is an act of last resort.