Armchair quarterbacking is dumb, and yet you seem to have this wrapped up into a neat little narrative. "Local street gangs"? I've seen footage from just after the shooting, and outrage amongst average people was already palpable. His body was left uncovered in the street for hours. Witnesses have said he had his hands up when he was shot, and he was definitely unarmed. Against a back drop of what seem to be regular incidents of police shooting unarmed African-Americans, it's only understandable that frustration spilled over into the streets.
This the kind policing the residents of Ferguson are deaing with:
http://www.thedailybeast.com/articles/2014/08/15/the-day-ferguson-cops-were-caught-in-a-bloody-lie.html#
The militarization of police is an issue in the US and Canada, but more often than not, the target of this type of overbearing use of authority in the US are minority communities. Just compare the response to these protests to the response to Clive Bundy and his militia friends. Those guys were definitely well armed, and openly threatened to use force against law enforcement who were attempting to remove cattle from public land. There are pics all over the place of these militia guys actually staring down their sights at federal agents. Now imagine what would have happened if the citizens of Ferguson had done the same and shown up to their protests carrying AR-15s. It's a clear double standard, and you're kidding yourself if you think it isn't noticed. Cops in the US (as recently revealed) have far more to fear from racist white conservatives packing serious weaponry than black gang members and Islamic extremists (both of which are used to justify the fact that cities of 21,000 "need" MRAPs).