In all the news and rioting, only last night did someone cite the MO law covering the use of deadly force by police. The statute says it's justified when an officer reasonably believes deadly force is immediately required to effect the arrest AND the officer has reasonable grounds to believe the person is wanted for a felony or has committed a felony.
Wow, that's HUGE. Good luck convicting the cop with that standard as the law.
The latter part is almost surely met on two separate basis. The two had just committed a felony robbery, they matched the description. And assuming the officer has injuries, witnesses that Brown hit him while he was in the car, that also meets it.
So the law says the cop doesn't even have to be in fear for his life, only that deadly force is needed to effect the arrest. By that standard, if I were a fat cop, sounds like I could shoot some felony suspect that's fleeing in the back, because that's the only way to effect the arrest.