The English jurist Lord Hewart said it best: 'Not only must Justice be done; it must also be seen to be done.' http://en.wikipedia.org/wiki/R_v_Sussex_Justices,_ex_p_McCar...
But there almost always is in reality, if there's a reasonable suspect. The only exception seems to be with law enforcement officers.
Often it never makes it that far. In many self-defense cases involving non-police civilians the district attorney will decide that no crime was committed and that will be the end of it.
One quick example from the top of a google search: http://www.jsonline.com/news/crime/janitor-wont-be-charged-i...
Not to mention plenty already "have their financial lives destroyed" through the Civil Forfeiture law.
At the state level it's much higher 3-4%. You can say it's a rubber stamp, but in practice the average is raised by the fact that most criminal cases are slam dunk (that kilo of cocaine found in the accused's car is almost certainly his). And the bar for an indictment is relatively low, just probable cause. It'd be pretty uncomfortable if prosecuto so we're bringing lots of cases where they don't even have enough evidence to convince 3/4 of a grand jury there is probable cause.
[1] The distinction is relevant because federal and state prosecutors handle totally different sorts of crimes.
That goes doubly when the killer is a law enforcement officer. If we want to fix the current problems with policing in America, the only way to do that is to hold LEOs to higher standards than everyone else (and ideally stiffer penalties).