Three different autopsies have been or are being conducted on the body of Michael Brown, the unarmed black teenager who was shot by police in Ferguson, MO on August 9th. The first of them was the autopsy conducted by St. Louis County — the jurisdiction conducting the criminal investigation into Brown's death, to see if Officer Darren Wilson should be charged with a crime. It included a toxicology test, which can only be done once.
Even though the results of the autopsy itself haven't been released to the public yet, a source with the county government told the Washington Post on August 18th that the toxicology test showed that Brown had marijuana in his system at the time he was killed. (This doesn't mean he was intoxicated when he was killed, just that he had consumed marijuana at some point in the last 30 days.)
This doesn't tell us much. There's no link between marijuana use and aggression (and even if there was, a general link wouldn't tell us about a specific case like this one). Moreover, the toxicology test doesn't have any impact on whether or not it was legal for Wilson to shoot and kill Brown. The legal standards for cops shooting civilians focus on the officer's behavior in the situation.
Anyone who says it's important that Brown had marijuana in his system when he was killed isn't making an argument about legality. They're making an argument about Brown's character. And that's simply not the question that St. Louis County is supposed to be answering right now.