It's not simply a possibility that Brown robbed the store, it's fact.
And Dorian Johnson is not a credible witness. He changed his story many times. He also has a criminal history for offenses involving dishonesty. There are only two reasons why I credit the portion of his testimony before the grand jury that Brown robbed the store. First, because that's the only reasonable conclusion one can reach in light of the security footage. Second, because he threw Mike Brown under the bus in order to separate himself from Brown's conduct and avoid being charged himself. Whether he actually knew Brown was going to rob the store is irrelevant to me -- the point is, he testified that Brown did not pay for the items (i.e., stole them), and pushed/shoved the store owner out of the way. The fact that you won't accept what the security footage clearly shows, and what is corroborated by Johnson's testimony, makes your overall position less credible. The fact that Brown was casually walking in the middle of the street following the robbery does not demonstrate that he didn't commit a crime. Remember - Dorian Johnson testified that Mike Brown stole the cigars, which supports what the security footage shows. It's not like Brown had just carried out an elaborate jewelry store heist, he simply ripped off some cigars from a convenience store and strong-armed the clerk. His walking in the middle of the street after doing so just demonstrates his tough-guy mentality and thuggish character. Also, remember, Brown was found with 2+ grams of weed on his person. If he was scared of being arrested, as you speculate, he certainly wouldn't have been walking around carrying drugs.
I would suggest that you read the transcripts from the grand jury proceedings for a better understanding of what actually happened and what the physical evidence shows. The media did not accurately portray what happened for both sides of the spectrum.
Edit: And as far as my views on police are concerned, I know there are good cops, and I know there are bad cops. I've had my fair share of encounters with both -- I wasn't the most responsible teenager growing up. I am in agreement that unfair racial profiling occurs, and that it may even occur more often than not in high-crime areas. But, in my opinion, the person being mistreated can avoid being killed in almost all situations. There are also number of avenues available to pursue claims against bad cops after the fact.