It's fair to say constitutional democracies struggle with the prosecution of their elected rulers. Naturally, rulers will break laws and treaties as they see fit; that is the point of having an executive -- not to mention a justice department which reports to it. But it's also tacitly understood that this will only be done on behalf of the "national interest," that is to say, the ruling class as a whole.
As I've already suggested elsewhere, what distinguishes the liberal democracy from dictatorship is that it aims to advance the rights of elites as a class, not just whatever portion enjoys majority control of the state. In America, this is the spirit of "bipartisanship": something that works for all rich people. In this respect, it is very important that government never serve as a seat of retribution against one's political rivals: this pledge is at the heart of every liberal democracy.
Torture is an interesting example. It's clearly prohibited in American and international law, but, like so many things, it only becomes objectionable when the costs are judged to exceed the benefits for elite concerns.
This is reflected nicely in the current "national debate" -- a debate over whether to enforce the law! -- with Republicans arguing that torture helps the republic by protecting it, and Democrats arguing that torture hurts the republic for miscellaneous reasons, including the notion that it "hurts our image around the world," thereby making the world less malleable to our interests.
(Of course, any random Middle Easterner suspected of something by US agencies who is subsequently detained and tortured would probably insist that the "image of America" is not the only thing harmed in the process, but that is not a concern which registers very high in the art of statecraft; as such, "harm to ourselves" -- to our very soul! -- appears to be the argument the Democratic Party prefers best.)
Whenever the ruling class does not enjoy consensus, the American people are treated to "debate," and solicited to support one side or the other in order to settle the concern. In this case, we have witnessed reluctance on the part of Obama start this process, presumably because it touches on principles that are dear to elites -- like the idea of holding elected officials to account for their actions after they are out of power. Suffice it to say, neither party wants to open this Pandora's box, and for very good reason. But it may be that the level of controversy this issue has stoked makes it impossible to get around; some low-level domestic prosecutions may be necessary.