The 20-year-old man had no firearm but his behavior gave the cop sufficient reason to feel threatened. Thats really all an officer needs to shoot your ass, in fact that's pretty much the only reason anyone needs to shoot someone else. That was why Zimmy got off too, he felt his life was being threatened. Same with Wilson. Brown didn't comply with commands, in fact aggressed against the officer and then tried to take his gun, probabaly so he could use it to kill wilson.
Think about it, If a policeman asks you to move to the sidewalk, do you feel its justified to walk up to the police vehicle and start punching that officer in the head and then try to take his gun? Does that seem like a normal reaction to you? Is that what you would do if an officer asked you to do something?