One of my first thoughts this is, perhaps, an appropriate essay to write a week after Easter, for Easter is the memorial of an act of torture and killing--and a claim to transcendence of pain and death, but that is harder to judge. It is one of the great ironies of history that the great religious figures consistently oppose torture in word and action, and yet major factions of their followers consistently fail to act on the words and examples. Why is that? Against the teachings, repeated over and over, against common sense, against practical experience, why does anyone defend or practice torture? It is a a special case of the question: "Why coerce?" but coercion can serve a purpose, however unhealthy; coercion is less immediately repugnant and useless.
That, though, contains one of the seeds of an answer; torture is a prime technique of oppression, and oppressors use it, because for that, it works. In an empire, where the government depends on the consent and support of the citizens, the rationalization of torture as necessary is itself necessary to maintain the power of the empire, and this is why it is rationalized in Iraq. In Iraq, though, it fails. Torture can cow a citizenry, but it cannot stop an insurgency; the only ways to do that are either to settle with the insurgents, or to deprive the insurgency of necessary resources, and the US military (like all competent militaries) is very well aware of this. So, why...?
In cowing a citizenry, there is more of an answer; torture is one of the techniques used to maintain a coercive social order. In a certain sense, it is one of only three techniques; ultimately all forms of oppression grind down to hurting people, depriving people, or killing people, and hurting people is by far the most effective. But why participate in oppression of your own people, then? Why defend torture? I think there are multiple ways to look at the problem, but in the end they all tell the same story.
One way to look t the problem, as I said, is that it maintains an unjust social order: torture is the tool of the tyrant. Another way to look at the problem, though, is to recognize that many people believe it maintains their place in the social hierarchy: they are superior to the people being tortured, who they regard as a threat. And, finally, it is the expression, ultimately, of ape tribal behavior: hurting each other is part of how apes maintain power relations within their tribes. One of the things that goes wrong in warfare is the failure to grasp that it is conflict between, rather than within, tribes. Most people will accept a lot of abuse from members of their own "tribe" but they will fight against outsiders. Thus, tyrants believe that the people they attack can be cowed as easily as their own people, whereas people outside their society will instead fight; this belief was clearly in evidence in the W. Bush administration's attack on Iraq, where the leaders seriously believed that a show of force would be sufficient to bring Iraq into line and found, as more knowledgeable strategists already knew, that it would be necessary to deprive and to kill.
So, finally, I believe torture is a policy is built on the combination of ape instinct and the logic of power. What, then, may we do to devalue it? Well, to begin with, we keep telling people what's wrong with it as policy. Over time, people can be persuaded. Understanding sometimes help; the knowledge that this impulse is not from "god" (or whatever) at all, but rather a blind instinct devalues the impulse. In this regard, it also helps to point to the many religious teachings that are very much critical of torture; these are usually part of the the "primary" sources of the religion, inasmuch as any statement that is written down long after the fact can be said to be primary. Encouraging empathy with the victims is also valuable. For times, at least, these things can be made part of the past. And, who knows? Maybe over time humanity can learn.