Brown ran a full city block away from the confrontation with Officer Wilson. Wilson had PULENTY of time to re-assess the situation per his training, note that Brown was unarmed and fleeing per his training, and change modes of response per his training. Everyone is focusing on the wrangling in Wilson's car, as though that's where Brown was killed. I agree that if an officer is trapped in his car fending off an attack, then he has very limited options and might justify using lethal force, especially when its CQB and there is now a struggle over the gun.
But ALL of that changed when Brown ran off, with only minor wounds to his arm or finger. Not ran off 10 feet, turned around and immediately came back at Wilson as he was attempting to get out of his car or something, not fully recovered from being attacked in the car. Ran off a full city block away, at least 150 feet by Wilson's own admission. The threat to Wilson was no longer applicable. He had PLENTY of time to reassess and deploy different approach to the situation, per his training. Instead, he just went after Brown and shot him down, knowing he was unarmed, knowing the threat continuum had been broken.