This latter question is at the center of the new Batman movie, The Dark Knight. It gives away nothing to say that Gotham's new district attorney is portrayed as a white knight (he's not the title character!). The other forces for good in the movie continually remind him that he has to keep his nose clean -- if anything happens to his reputation, the city will lose hope and all the bad guys he has put away will go free.
Leaving aside the melodrama of an entire city's mood depending on its good feelings about the D.A., clearly this is one of the elements of the movie that makes no sense if you stop and think about it. No matter how loudly you complain about a prosecutor or his motives or even his actions, that doesn't negate the indictments he has scored in front of a grand jury or the convictions she's exacted from a regular jury.
I mean, sometimes prosecutors end up disgraced and have to resign. Some might even end up imprisoned themselves. But that doesn't lead to automatic issuance of get out of jail free cards for all the people they've put away.
I point all this out even as I recognize that it's silly to complain that any movie, particularly one about a comic book hero, bears a shaky resemblance to the rules governing real life.
