"Hands up" doesn't mean "no threat." If a 300 lb giant is charging me, regardless of where his hands are, I would feel threatened. And then if he's already tussled with me, maybe going after my gun, and maybe breaking a bone in my face, I'm going to consider him a thread. That's why there is all the speculation about the shot that killed him and the angle of that shot in his head.
This case will likely turn on the few seconds that Mike Brown turned around and faced the officer and what happened next: moving forward, not moving forward, threatening. That's why there is all the speculation about the angle of the shot in his head that killed him.
We saw Mike Brown threaten a man 1/3 of his size just a few minutes before he was shot. Brown used his giant bulk to threatened and intimidate that guy, it was his MO.