Paul Krugman says there is really only one reason why the Bush administration is so determined to torture people:
To show that it can.There is another reason too, of course: to intimidate opponents, both domestically and internationally.
The central drive of the Bush administration — more fundamental than any particular policy — has been the effort to eliminate all limits on the president’s power. Torture, I believe, appeals to the president and the vice president precisely because it’s a violation of both law and tradition. By making an illegal and immoral practice a key element of U.S. policy, they’re asserting their right to do whatever they claim is necessary.
But I doubt the Bush Boyz have thought it all through that carefully. They just do this sort of shit out of habit. It's how they got where they are today.