here is an established protocol to wait for assistance upon coming into a situation when you come upon an individual or a group of individuals that is being extremely aggressive
The officer really didn't have the luxury of waiting for back-up. Michael Brown & his friend started running away. At that time, Officer Wilson was pretty sure that he was the one who had just robbed the store (he fit the description and had the cigars in his hand) and as a trained cop, he did what he was trained to do. Michael Brown appeared to be very aggressive (believable after seeing the video of him robbing the store and shoving aside the worker who tried to stop him) and if he realized that he was facing arrest, it's not too difficult to imagine his response. He was a very big guy too, 6'3" and nearly 300 lbs. He hardly needed a gun to be seen as dangerous, especially as he had already attacked Officer Wilson.
We all have 20/20 vision in hindsight. Officer Wilson wasn't give that luxury; he had to make a decision in probably less than 5 seconds. The tragedy is that none of this needed to happen. Had Michael & his friend not robbed the store, this wouldn't have happened. If he hadn't been acting like a smartass walking down the middle of the street, refusing to move when ordered to by Officer Wilson, this wouldn't have happened. His own actions, unfortunately, cost him his life that day and now his parents and family have to go the rest of their lives without their son. We can only hope that maybe this tragedy might be a lesson for other teenagers who might think they're above the law. It can cost them everything.