Drum has a powerful post on why we don't allow torture:
I don't care about the Geneva Conventions or U.S. law. I don't care about the difference between torture and "harsh treatment." I don't care about the difference between uniformed combatants and terrorists. I don't care whether it "works." I oppose torture regardless of the current state of the law; I oppose even moderate abuse of helpless detainees; I oppose abuse of criminal suspects and religious heretics as much as I oppose it during wartime; and I oppose it even if it produces useful information.
The whole point of civilization is as much moral advancement as it is physical and technological advancement.
But that moral progress comes slowly and very, very tenuously. In the United States alone, it took centuries to decide that slavery was evil, that children shouldn't be allowed to work 12-hour days on power looms, and that police shouldn't be allowed to beat confessions out of suspects.
On other things there's no consensus yet. Like it or not, we still make war, and so does the rest of the world. But at least until recently, there was a consensus that torture is wrong. Full stop. It was the practice of tyrants and barbarians. But like all moral progress, the consensus on torture is tenuous, and the only way to hold on to it the only way to expand it is by insisting absolutely and without exception that we not allow ourselves to backslide. Human nature being what it is savage, vengeful, and tribal the temptations are just too great. Small exceptions will inevitably grow into big ones, big ones into routine ones, and the progress of centuries is undone in an eyeblink.