The fastest way to help someone understand why torture is unacceptable as a means to an end is this one sentence:

If we believe that tortue works and that we as a nation will engage in torture because it works….well, then we must accept that each and every last one of our enemies in each and every previous war with our country were justified to torture our captured soldiers since we believe it works too.

On this issue, a nation cannot have its cake and eat it too – either you believe that torture is unacceptable and make no exceptions, or you believe it is acceptable and that its use is then acceptable when it’s our soldiers being tortured.