In the rioting and other snarling about the Ferguson prosecutor's failure to obtain an indictment (or goad the grand jury into returning one), I have yet to hear, even once, a quotation I otherwise see repeatedly, especially from those proclaiming themselves civil libertarians.  I thought I would dust it off just to jog their memories:

The [prosecutor] is the representative not of an ordinary party to a controversy, but of a sovereignty whose obligation to govern impartially is as compelling as its obligation to govern at all; and whose interest, therefore, in a criminal prosecution is not that it shall win a case, but that justice shall be done.

Berger v. United States, 295 U.S. 78, 88 (1935)