Such deaths are intolerable. Debates over police conduct often pit the right against the left, but the conviction that no one should die on the basis of a hunch should be trans-partisan. It is also wise to feel a certain mistrust of government power exerted against the citizen; a certain fear that agents of the state may overstep their authority or use well-intentioned but destructive zeal; a certain expectation that the legal institutions to which they are accountable may fail to hold them to account; and a certain recognition that rules defining the rights of citizens and the scope of government power against them may be vague or elastic in ways that can harm the individual. That the individuals at issue here have often violated the law may make them harder to sympathize with, but for this very reason it is proper to wonder whether the insecurity of their rights is too readily tolerated. After more than a year of debate and protest and occasional riots in response to particular police killings, it would be well to take a scrutinizing look at use-of-force rules themselves.

There is a moral logic common to such rules. Two requirements must be met for a use of force to be justified: a “proportionality” requirement and a “necessity” requirement. The proportionality requirement, as Paul H. Robinson, an expert on criminal law at the University of Pennsylvania Law School, summed it up for me, concerns whether a person posed a danger to which the force used against him was a commensurate response. For deadly force, the usual standard is that someone must present a threat of death or serious bodily injury. The necessity requirement concerns whether, at the time when deadly force was used, it was truly needed to prevent those threats from being realized. If someone makes a death threat, for example, he has satisfied the proportionality requirement, but that doesn’t mean the police may shoot him dead at once. Deadly force becomes permissible only when and if it is needed to keep him from killing.

As an abstraction, this makes sense. In practice, it can be vexingly indeterminate. A use of deadly force is, after all, preventative and the public’s judgment of it retrospective. What would definitely show it to have been justified is an affirmative answer to the question, “If deadly force had not been used against this person, would he have gone on to inflict death or serious injury?” And the answer to that question is unknowable. Sometimes the public may feel confident enough saying yes. But suppose it says, “Maybe. Who knows?” Is that reason enough to have ended someone’s life?

​* * *

When Michael T. Slager, a police officer in North Charleston, South Carolina, shot Walter Scott dead, there was universal agreement that the act was an outrage. Scott had been unarmed and running away from Slager when Slager fired eight rounds at Scott’s back. No one could have argued that using deadly force had been either proportional or necessary. But Scott’s case was perhaps uncommonly clear-cut. The killing of Milton Hall illustrates the ambiguities that can arise in applying a use-of-force standard, and the ways in which those ambiguities can lead to horrifying results.