As you're already aware, police kill people all the time and are basically never punished for it. In fact, they're rarely indicted, as prosecutors rarely bother even filing charges against police because they know they won't stick. Humorously enough, one of the major reasons it's difficult to bring charges against police is because the police, like all violent and lawless gangs, will absolutely never testify against one another no matter what has been done. Protecting your murderous/rapist/thieving friends in blue means far more than the law to these people.
Darren Wilson, the murderer who is now walking free, was basically uninjured. Only in a nation with such a vile history of racism could barely-visible bruises somehow equate to the necessity of murdering an unarmed child. In fact, Wilson specifically cited the classic "big nigga" defense to justify his actions; even though he and Brown were about the same size, Brown is Black, and Black people have magical abilities which make the stronger than white people and impervious to pain. This has actually been extensively studied, as white people think Black people are such magical beasts that white people think pain doesn't effect Black people to nearly the same extent. This is why Wilson needed to murder Brown; because he was a magical Black man who wouldn't feel pain so he had to be murdered.
The fact that Wilson was not indicted means nothing about his innocence, but instead says everything about how the county prosecutor did not want to go to trial. A grand jury not returning any indictment at all is actually pretty fucking rare. To quote the linked article:
Former New York state Chief Judge Sol Wachtler famously remarked that a prosecutor could persuade a grand jury to "indict a ham sandwich." The data suggests he was barely exaggerating: According to the Bureau of Justice Statistics, U.S. attorneys prosecuted 162,000 federal cases in 2010, the most recent year for which we have data. Grand juries declined to return an indictment in 11 of them.
It's also important to remember that the "physical" evidence, which is now being held in much higher esteem than the witness evidence (mostly because the witness evidence all pointed to Wilson being a murderer), is pretty much worthless. In fact, a recent meta study by the National Academy of Sciences found that pretty much every forensic science measure which isn't based on direct DNA evidence is completely worthless. There are basically no standards, no accreditation, and it's all pretty much make believe. Hell, one recent article I read gave top forensic scientists hair to analyze and these "experts" couldn't even tell if the hair was real or fake, let alone whether it came from a human or an animal. These are the people whose expertise was relied on for releasing Wilson.
One of the reasons you'll see so many clueless white people saying stupid shit about this is that most white people don't actually know or speak to any people of color. So for a lot of white people, this is some weird, isolated incident and they can't understand why "those" people are so upset. Hint: it's from the fact they are hunted like animals by the very people paid to protect them.
Finally, if you're one of those people who sympathizes but wonders why people would go around burning down buildings in "their neighborhood," I suggest you read this. Or just listen to Malcolm, which is always good advice, but especially pertinent here:
A good example of how they do it in New York: Last summer, when the Blacks were rioting—the riots, actually they weren't riots in the first place; they were reactions against police brutality. And when the Afro-Americans reacted against the brutal measures that were executed against them by the police, the press all over the world projected them as rioters. When the store windows were broken in the Black community, immediately it was made to appear that this was being done not by people who were reacting over civil rights violations, but they gave the impression that these were hoodlums, vagrants, criminals....
But this is wrong. In America the Black community in which we live is not owned by us. The landlord is white. The merchant is white. In fact, the entire economy of the Black community in the States is controlled by someone who doesn't even live there. The property that we live in is owned by someone else. The store that we trade with is operated by someone else. And these are the people who suck the economic blood of our community.