Just wanted to point out that yes there are cases where unarmed folks are a real threat (Chuck Norris et al, or the severely deranged or psychotic, say due to substance abuse).
Chuck Norris is a cute reference but in reality there are a nearly infinite number of factors that come into play. Gender, size, training, existing injuries, number of opponents, and so on. A fight might even start out as a normal fisticuffs where deadly force would not be permissible but escalate to a situation where the defending party is too injured to continue to defend themselves without resorting to deadly force. If you beat the snot out of me to the point that I'm about to pass out I'm well within my rights to shoot you, since I'll no longer be able to defend myself whilst unconscious. If you come after me with three of your friends I'm well within my rights to resort to deadly force, since even a well trained individual is not likely to prevail against 4 to 1 odds. Ditto if you've got 200 pounds on me. The relevant term is "disparity of force" and it seems to have applied in the incident in Ferguson.
But mostly the mere fact that LEO are armed should be sufficiently threatening to subdue and solicit cooperation of unarmed individuals.
Which is what happens the lion's share of the time. Cases where the suspect surrenders without a fight don't make the news though.
Second, the Grand Jury system as I understand it is a bit of a quirky thing, discarded in most places except the U.S. It was meant as a protection of the public from excessive use of executive power, but arguably used in cases such as Ferguson to the opposite effect. Why not simply a regularly prosecuted case?
Grand Jury is part of the prosecution in the United States. Before you can be prosecuted for a felony offense the Government needs to secure an indictment against you. This is the job of the Grand Jury. The standard they must meet is significantly lower than a Petit Jury at trial, which must find beyond a reasonable doubt with a unanimous ruling that you committed the crime in question. The Grand Jury need only find that there's probable cause you committed the crime in question. They do not need a unanimous ruling to do this, simply a majority of the empaneled Grand Jurors must vote for indictment. The State is allowed to introduce evidence at Grand Jury that it would not be allowed to use at trial, like hearsay or illegally obtained evidence, and the deck is further stacked against the defendant in that if he chooses to testify he waives his right to be represented by counsel and his right to refuse to answer questions that might incriminate him.
If the State can't meet this simplified burden of proof under rules that greatly favor it then what is the point of preceding to trial? More than 90% of cases presented before the Grand Jury result in indictment. The remaining cases are so exceedingly weak that there's less than zero chance they would result in conviction at trial were the Grand Jury system to be abolished. Additionally, it's mandated by our Constitution, so the process of abolishing it is not a simple one.