I hate cops as much as anyone, I want cameras on them, but I'm not throwing Darren Wilson under the bus over it. There is corroborating evidence that Brown grabbed his gun (with the caveat that all of this is leaks, and technically we don't know any evidence): an autopsy report concluded that a gunshot to the hand was at close range (because of gun powder right by the wound itself?), and 6 black witnesses have told the grand jury that Brown reached into the vehicle and started the violent altercation. I don't know anything else, I'm not saying Wilson was right or wrong for firing shots after that, but there is evidence for Brown starting the altercation and trying to grab the gun.
Your only logic is that cops lie so stop trusting them, which really just amounts to cops not being allowed to kill when their life is in danger, because then you'll just assume it was murder and ignore their explanation (and forensic and eyewitness evidence that doesn't fit your predetermined view of the officer). Whether Wilson is a criminal or not, Michael Brown is a criminal for robbing a store and attacking an officer. He does not deserve the presumption that he was acting reasonably and innocently after initiating all of the violence in the first place (and he's not on trial, so f*ck the presumption of innocence, Wilson gets the presumption of innocence because he's the one whose freedom is on the line now).