The issue of what actually happened is interesting, but almost irrelevant to what is going on there now. The protesters seem to believe (and not without some justification, given the actions of the Chief of Police), that had the officer in question walked into the store and emptied his gun into Brown's back the cops would still be covering for him. They don't trust the authorities on this one, and the police haven't gone out of their way to gain trust either.
As to the politics of the issue, some people here might want to expand their reading a bit. While there are some on the right who are defending the cops, there are others who are talking about how cops now seem to see themselves as soldiers and not law enforcement officers. That change alters the way they interact with citizens in a way that is unhealthy for a free society.