Why is he dirtbag? Is he a dirtbag for the confrontation with police after the incident? (I think so...)
Considering the issues with that department and the community some might disagree. Frustration in some communities with policing had led to fucked up dynamics. Where neither side is on the right side of the law.
"Dirtbag" is irrelevant to the facts of the matter. There is no redeeming behavior on Brown's part, name calling isn't useful.
What we have determined is that some action caused Brown to be subject to confrontation by a police officer. Note we are discussing something between two individuals at the moment.
One side has contended that Brown was executed, another that Brown presented himself as a threat and the officer responded appropriately. Both can't be correct.
Fortunately we had Holder the AG come in and launch an investigation which would not be in the pocket of the police or state LEO. In examining the evidence the conclusion was reached that the officer's account was consistent with the facts and the other various statements were not. In other words there was no factual basis for action against the officer.
But you are correct, the department as a whole was found to be racist and that naturally does not sit well. With me at least nothing excuses the attacks against the officer based on belief of "what should have been" and a career ruined.
Getting back to Brown, the fact that there was a different explanation for his actions in the shop is wholly disconnected to his response when conducted by the officer.
It sucks, really sucks that things went down as they did, but there is no demon involved, just demonization.