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.
There are not any very clear cut rules for when deadly force may be used. There are rules, but they are, by necessity, rather vague. In addition, there are different rules that apply to the actions of police officers on the one hand and private citizens on the other.
The basic rule for use of deadly force by police officers is that the circumstances must be such that it is "reasonable" to use deadly force. In other words, the harm that the officer is trying to prevent must be great, and there must not be any other reasonable options available to him or her. This is, quite clearly, a very murky standard since there is no objective way to define either of these ideas.
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