Actually, the Israelis did just that, back in '94, after Palistinians kidnapped Nachshon Waxman. His captors killed him during the attempted rescue, but the way the Israelis found out where he was being held was through "coercive interrogation" that went beyond government guidelines. In other words, they tortured the driver of the car used in the kidnapping, and got accurate information.
Saying all that, as Jmax points out below, the times when the scenario is true are few and far between. In those cases, it is best that the people involved be acting outside the guidelines and illegally - they might be pardoned in light of the circumstance. But we've seen what happens when what should be extraordinary is treated as ordinary.
(no subject)
This article in the Weekly Standard mentions the above.
Another interesting link:
NYtimes
Saying all that, as Jmax points out below, the times when the scenario is true are few and far between. In those cases, it is best that the people involved be acting outside the guidelines and illegally - they might be pardoned in light of the circumstance. But we've seen what happens when what should be extraordinary is treated as ordinary.