Bush could Pardon in Advance Torture Interrogators
Bush is trying to say that CIA interrogators won't do their job without protection from legal liability due to torture.
If they really did have a significant terrorist who had critical information, Bush could just issue an advance written pardon to the interrogators if torture was really and truly called for, as in the hypothetical example of a terrorist who knows the location of a nuclear bomb hidden in a city.
But this is not good enough for Bush. Why? Because then Bush himself would be liable and responsible and on record. In addition, Bush wants the privilege of ordering torture on a wide scale by entire government departments - he doesn't have time to issue pardons for all the low-level people he'd like to have committing torture.