Manzi asks the question. I approach this from the just war tradition in which war, however vile, is sometimes defensible against a greater evil. Torture, however, is never moral or defensible under any circumstances. Why? It has to do, I believe, with autonomy. An enemy soldier that you are battling in combat remains autonomous (and potentially dangerous) until the moment of capture or surrender. At that point, his autonomy ends, as he is in captivity, unable to cause you further harm. And the infliction of severe pain or violence on someone who is thereby defenseless carries a much deeper moral weight than a fair or even unfair fight.
We all know this intuitively. It is the difference between two boys duking it out on a playground and a gang of boys restraining one while another beats the crap out of him. Torture is a form of cowardice and a form of cruelty, which is inherently different than the sometimes necessary evil of just warfare. My best attempt at expaining the relationship between torture and freedom, and why torture can only endure in unfree societies, is from 2005:
Torture is the polar opposite of freedom. It is the banishment of all freedom from a human body and soul, insofar as that is possible. As human beings, we all inhabit bodies and have minds, souls, and reflexes that are designed in part to protect those bodies: to resist or flinch from pain, to protect the psyche from disintegration, and to maintain a sense of selfhood that is the basis for the concept of personal liberty. What torture does is use these involuntary, self-protective, self-defining resources of human beings against the integrity of the human being himself. It takes what is most involuntary in a person and uses it to break that person's will. It takes what is animal in us and deploys it against what makes us human.
As an American commander wrote in an August 2003 e-mail about his instructions to torture prisoners at Abu Ghraib, "The gloves are coming off gentlemen regarding these detainees, Col. Boltz has made it clear that we want these individuals broken."
What does it mean to "break" an individual? As the French essayist Michel de Montaigne once commented, and Shakespeare echoed, even the greatest philosophers have difficulty thinking clearly when they have a toothache. These wise men were describing the inescapable frailty of the human experience, mocking the claims of some seers to be above basic human feelings and bodily needs. If that frailty is exposed by a toothache, it is beyond dispute in the case of torture. The infliction of physical pain on a person with no means of defending himself is designed to render that person completely subservient to his torturers. It is designed to extirpate his autonomy as a human being, to render his control as an individual beyond his own reach. That is why the term "break" is instructive. Something broken can be put back together, but it will never regain the status of being unbroken--of having integrity. When you break a human being, you turn him into something subhuman. You enslave him. This is why the Romans reserved torture for slaves, not citizens, and why slavery and torture were inextricably linked in the antebellum South.
What you see in the relationship between torturer and tortured is the absolute darkness of totalitarianism. You see one individual granted the most complete power he can ever hold over another. Not just confinement of his mobility--the abolition of his very agency. Torture uses a person's body to remove from his own control his conscience, his thoughts, his faith, his selfhood.