However the later fatal shots should in my view be treated as a separate incident and judged on its own merits - saying "shooting him is OK because he attacked a policeman earlier" is not acceptable in my view.
Neither the is objective measure. For instance, instead of being attacked in his patrol vehicle, Brown waits patiently to be detained while Wilson uses unlawful force -- let's say he came out of the car to meet a compliant Brown with baton strikes. That fact alone isn't technically dispositive, but may evidence some (profound) relevance to a finder of fact for any subsequent claim.
As far as the basis of the claim of self defense justification, Wilson is evaluated from his perspective at the time of the act(s) for which he claims that justification -- i.e.
knowing what he knows.
In theory they (justifications of self defense) are evaluated in the same way as a civilian claim, though in reality the nature of a LEO's duties creates important distinctions. For instance, the concept of innocence -- that is (generally) the force cannot be provoked by the person later claiming justification (often as well as the result of criminal activity). If
Darryl Wilson, a civilian, reverses to block Brown's path it would be argued as provocation.
The same applies to avoidance, the civilian has the responsibility to avoid conflicts that could lead to a claim of justification to varying degrees. A LEO is often tasked to be the actual agent of conflict. The hard question in the civilian analog is how can one claim to be in fear of death or bodily injury when they chased the guy down the street with a gun yelling for him to stop.
However, just as in a civilian analog, Wilson is allowed benefit of the knowledge he had just acquired regarding Brown's motive, intent and peacefulness.
Also remember that Wilson has to justify shooting Brown in the car in the same manner he has to justify shooting him in the street.
