The line is drawn at the points where a reasonably cautious and prudent person, in the same circumstance as Wilson, would be in fear of death or serious physical injury, feel the threat was immediate and feel the use of force was necessary to prevent death or serious injury; or -- since Wilson was a law enforcement officer presumably attempting a lawful detention -- at the point where a reasonably cautious and prudent person would feel the need to effect Brown's arrest without delay in order to prevent death or serious injury to himself or others and feel the use of force was necessary to effect the arrest.
If Brown surrendered and was compliant, the use of force was most probably unlawful.
If Brown surrendered and was not compliant, but did not give rise to fear of death or great bodily injury, the use of force was probably unlawful.
If Brown surrendered, then started exhibiting aggressive behaviors after have just assaulted and injured a police officer, the use of force will probably be viewed as justified.
There's a reason that Wilson hasn't been charged. Again, I suggest that after/if the actual police statements are released by the court, eyewitness testimony isn't going to be the gold standard for reliability that we see here today.