What I am speaking from are the facts that have already come out to the public from the mouth of the Chief of Police of Ferguson.
Brown broke off and ran away from it. Why chase him down and shoot then???
You do yet not know if Brown turned around and started coming back. Also you do not know if Brown was on drugs that would have made him more dangerous. That is why it is important to wait for the toxicology report and the coroner's findings as to whether Brown was shot in the front or the back.
Oh, and you don't know the size of the officer in question, so you have no idea if it was an evenly matched wrestling situation. He would have to be pretty large to be in Brown's weight class.