The problem with deadly force situations arises in that the officer is almost always required to make a judgement call as to the degree of danger in a given situation within a very short period of time, often mere seconds. Because of this, it is quite difficult for a government, court or review board to fairly and consistently apply a set of criteria to such situations.
As a rule of thumb as opposed to a rule of law, the use of deadly force is considered justifiable if there was reasonable belief on the part of the officer that harm to themselves or to others was imminent. A teen on the street pointing a BB gun at pedestrians might result in the use of deadly force, and while a BB gun does not present an imminent threat to life, it is reasonable that an officer would judge it to be such a threat.
West's Encyclopedia of American Law defines justification for deadly force as:
For deadly force to be constitutional when an arrest is taking place, it must be the reasonable choice under all the circumstances at the time.
There's no way that can be evaluated on anything other than a case by case basis, nor can such reviews be based on hindsight. Instantaneous judgements make that impossible.