Scott Horton puts it simply:
Let's first focus on this question: Why is this evidence being destroyed? The answer is painfully acknowledged. The CIA leadership and other senior administration officials are fully cognizant of the fact that the use of a number of specific practices which these tapes almost certainly document, to-wit: waterboarding, long-time standing, hypothermia, psychotropic drugs and sleep deprivation in excess of two days, are serious crimes under American law and the law of almost all nations. Consequently, those who have used them and those who have authorized their use will almost certainly ultimately face criminal prosecution at some point in the future.
The Administration's attempts to immunize the perpetrators have failed. Any purported grant of a pardon by President Bush will be legally ineffective, because Bush himself is a collaborator in the scheme. And there is no statute of limitations. Therefore the prospect of prosecution is hardly far-fetched. It is a virtual certainty. So the evidence is being destroyed precisely because it would be used as evidence of criminal acts in a prosecution of administration figures and those acting under their direction. Therefore, this is a conscious, calculated obstruction of justice.
This is Mukasey's moment. He needs to find who authorized this and prosecute them. And if the president authorized it, obstruction of justice in the case of war crimes is obviously an impeachable offense.