Torture is wrong because it doesn't work. Societies must, at times, walk into questionable moral territory, but when that happens, you should at least get results. With torture, you don't. The confessions obtained are no good, and the enemies you create in the process will be highly motivated, and knock back at you hard.
First point, torture is useless: In the age of instant communication, whenever an Al Qaeda operative is picked up and detained, you can be sure that every plan involving that detainee will be changed within a few moments of his detention. These days, changing strategy wouldn't even require a phone call about the detention. A twitter between comrades will do the trick. Indeed before you have time to muster your most menacing voice and start waving cattle prods in someone's face, you can be sure that word has gone out about the arrest. These days, no act of war takes place in an outpost. Nothing takes place in the dark. They know what we're doing, and will adapt according to whom we've picked up and questioned.
Here's another reason why torture doesn't work: All that information you need so badly that you're willing to torture to get it? Well, that information will be altered by the fact of your torturing. By torturing, you've changed people's plans, made new enemies, and motivated new sectors of a society to take action against you. When you torture someone, you want information: "Tell us about your friends. What are they planning?" Well, by torturing someone, you've just created more of such people. Torture creates an entire family of new believers ready to die to avenge the tortured. So anything you might learn from such a venture is already outdated. By torturing you've brought others into the loop, created more flashpoints, more potential destruction, more people you'll have to arrest, and more people who'll want to get back at you at any cost. Torturing may not create armies, but it motivates them, motivates armies of brothers and sisters, nephews and cousins of the tortured, ready to do whatever it takes to bring you to your knees.
The reason why unleashing such emotionally-charged fresh enemies is such a bad idea is because in today's world, war has no "front," no "line of battle." What is war today? War amounts to facing the realization that any motivated person with a physics degree and a little luck can blow up a city of his choosing, any time, day or night. That means the only honest goal of warfare is to try to limit the number of people who are angry enough to want to do that. Torture creates more of them.