This defense is unpersuasive insofar as it narrows the analytic focus to the moment when the police officers pulled their triggers. Even if we give the officers the benefit of the doubt and assume that Harrison did lunge at them with the screwdriver, it seems to me that they made significant errors before and after that moment.

When they first saw Harrison fiddling with the screwdriver in a non-menacing way, why did they escalate the situation by shouting at the mentally ill man? They could have immediately backed up, giving themselves the time and space to try to deescalate. Why did they stand their ground instead, yelling with guns drawn? And after they shot Harrison five times, so that he lay face down on the ground bleeding and either dead or unconscious, why did they keep shouting at him to drop the screwdriver, as if he still posed a threat, rather than trying to save his life?

Understandably, the police officers involved aren't talking to the media. We don't know how they would answer these questions. Perhaps after looking at the video they are as convinced as you or me that they should have handled the situation better or that they ought to have been trained and acculturated to react differently.

But Jake Rouse, the policeman defending them in Facebook comments, goes on to express a more troubling attitude that I've seen elsewhere in law enforcement circles. This is the exchange that made me glad that he isn't a police officer in my city:

With the caveat that not all police officers think this way, Clint Nelissen is absolutely right—and Jake Rouse has a factually inaccurate understanding of the risks that he faces. Jobs where Americans get killed at a much higher rate than police officers include loggers, fishermen, aircraft pilots, roofers, steel workers, refuse collectors, power-line workers, truck drivers, agricultural workers, and construction laborers. That isn't to dispute that being a cop is a dangerous job compared to many or that even one dead or seriously wounded police officer is too many. But the comparison to soldiers in Iraq is wrongheaded in almost every respect and particularly absurd in the context of a mentally ill man with a screwdriver.

Eugene Robinson once pointed out that U.S. police officers shoot somewhere between 500 and 1,000 people per year, whereas "there were no fatal police shootings in Great Britain last year. Not one. In Germany, there have been eight police killings over the past two years. In Canada—a country with its own frontier ethos and no great aversion to firearms—police shootings average about a dozen a year." He added that this is partly because the U.S. is a gun-filled culture, but that something else was going on too. Since every developed country has both mentally ill people and screwdrivers, this case is a data point in support of that contention. Compare the video from Dallas to London policemen going above and beyond what anyone could reasonably expect in an attempt to disarm a man with a machete: