The law needs to be changed
The root of the problem is the law which gives police leeway when they fire their weapon.
There are major legal, institutional and social impediments to prosecuting police. Thousands of officers are involved in shootings every year, resulting in about 400 deaths annually. However, successful criminal prosecution of a police officer for killing someone in the line of duty, if no corruption is alleged, is extremely rare. Even when officers are convicted, the charges are often minimal. For example, Coleman Brackney, a Bella Vista, Oklahoma, police officer who was convicted of misdemeanor negligent homicide in 2010 after shooting an unarmed teen to death while in custody in his cruiser, went on to rejoin the police and was recently appointed chief of police in Sulphur Springs, Oklahoma.
http://america.aljazeera.com/opinions/2014/11/ferguson-police-misconductdarrenwilsongrandjury.html
The Brown case would be far from the most outrageous incident involving a police officer not being criminally charged for killing an unarmed person. In 2012, for example, Brian Claunch, a wheelchair-bound double amputee living in a group home in Houston, became unruly. After the cops arrived, Claunch, who had a history of mental illness, verbally threatened them from his wheelchair and waved a shiny object—a ballpoint pen. After Claunch refused to drop the pen, one of the officers shot him in the head, killing him.
Is it shocking the officer wasn’t charged? Yes. Unexpected? No. As The Texas Observer noted, between 2007 and 2012, Houston police officers shot and killed 109 people and injured another 111. How many of these shootings were deemed unjustified? Zero.
Claunch was white. I mention his race only because white people should, too, be concerned with being shot by law enforcement. In fact, the police have killed more whites than black people in recent years. But those numbers don’t paint the full picture. On a percentage basis, blacks are being shot and killed by the police in much higher numbers.
For example, as Mother Jones noted, between 2004 and 2008, Oakland police officers shot 37 people. How many were black? All of them. And even though in 40 percent of the cases the suspect was unarmed, not one police officer was charged with a crime. And Oakland is not unique here—similar numbers can be found in other big cities.
http://www.thedailybeast.com/articles/2014/08/26/why-killer-cops-walk-free.html
Wilson said to friends that he shot to defend his life. Deadly force is justified if the officer reasonably believed at that moment that he or others were in imminent danger, and it doesn’t matter whether any danger actually existed.
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“The reality is that the police often use excessive force, including sometimes deadly, and are rarely held criminally or civilly liable — and most police departments have no meaningful internal or external accountability mechanisms,” said Anthony Rothert, legal director of the American Civil Liberties Union of Missouri Foundation.
http://www.washingtonpost.com/politics/current-law-gives-police-wide-latitude-to-use-deadly-force/2014/08/28/768090c4-2d64-11e4-994d-202962a9150c_story.html
Tennessee V Garner
Tennessee v. Garner, 471 U.S. 1 (1985)[1], was a case in which the Supreme Court of the United States held that under the Fourth Amendment, when a law enforcement officer is pursuing a fleeing suspect, he or she may use deadly force to prevent escape only if the officer has probable cause to believe that the suspect poses a significant threat of death or serious physical injury to the officer or others.
http://en.wikipedia.org/wiki/Tennessee_v._Garner
The Missouri statue has a little of both.
(2) When he reasonably believes that such use of deadly force is immediately necessary to effect the arrest and also reasonably believes that the person to be arrested
(a) Has committed or attempted to commit a felony; or
(b) Is attempting to escape by use of a deadly weapon; or
(c) May otherwise endanger life or inflict serious physical injury unless arrested without delay.
http://lawofselfdefense.com/statute/mo-563-046-law-enforcement-officers-use-of-force-in-making-an-arrest/
I'll note in my hometown there were 2 cases that I was aware of where they best victims could do was a civil suit where they both lost. There are likely other issues like juries placing high value on the testimony from the cop but I feel the problems will only continue to persist if you remove this wide latitude. I wouldn't know how to go about doing this except to write your legislators but there is a big problem when misconduct charges are extremely rare if they fire their weapon.