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The latest developments in two bruising tragedies involving Twin Cities police illuminate once again a critical public-policy need few would question — to ensure that those who wear a badge and carry a gun are men and women of exceptionally sound judgment and steady nerves.

In fact, the large majority of police officers meet those exacting expectations.

But recent months have also brought a new flurry of cases in which Minnesota labor arbitrators have overruled police chiefs and sheriffs trying to discharge wayward officers — rare officers who, in their superiors' judgment, fall short.

This second-guessing exposes once again a union-driven disciplinary system that too often prevents law enforcement leadership from upholding the high standards of self-possession and reliability we need in our cops.

That there's no formula for the split-second, life-or-death choices police must make becomes clear in the strikingly different judgments authorities have made regarding the deaths of Jamar Clark and Philando Castile.

In October, Minneapolis Police Chief Janeé Harteau announced that the two Minneapolis patrolmen involved in Clark's 2015 shooting would face no department discipline. They'd already been cleared of criminal wrongdoing by both county and federal prosecutors. But Harteau made it clear that their actions when their attempt to arrest Clark led to a struggle for one officer's gun also complied with department policies and standards, despite the sorrowful outcome.

By contrast, in mid-November Ramsey County Attorney John Choi announced a very different decision in the July killing of Castile. Choi charged St. Anthony Officer Jeronimo Yanez with second-degree manslaughter, alleging that the officer's decision to shoot Castile during a traffic stop, after Castile had announced he was carrying a permitted pistol, was altogether unreasonable and unjustified.

In both encounters, events unfolded rapidly, swiftly erupting into deadly violence. A gun was involved in both situations, and in both, the police — before any shots were fired — made a quick judgment call to use deadly force.

Yet in one case the officers have been fully exonerated, while the other yielded an unusual decision to put a cop on trial.

One implication is hard to miss: The line separating a proper emergency resort to force and criminal negligence can be surprisingly fine. The unpredictable, chaotic nature of the dangers cops face probably means that preventable tragedies will be prevented only to the extent that we succeed in removing or deterring from police work those who aren't cut out for it — those who lack either the extraordinary courage or the extraordinary self-control such crises demand. (Which is most of us, it should be admitted.)

But the reality is that leaders of law enforcement agencies often don't succeed in ending the careers of cops when misconduct undermines superiors' confidence in their fitness for police work. That's thanks to a disciplinary system typically established in union contracts. It allows arbitrators chosen jointly by unions and managements to second­-guess the discipline decisions of police chiefs and sheriffs.

Consider three arbitration rulings from around Minnesota in recent months:

October: A Ramsey County deputy, off-duty and in a drunken rage (after being shown the door of a bar for "behav[ing] inappropriately with female patrons"), had delivered a cruel beating to his partner dog, captured on a security camera. An arbitrator overturned his firing, saying he deserved "another (but last) chance."

October: Responding to a domestic disturbance, a Minneapolis officer had twice pushed a woman to the ground with what an arbitrator agreed was excessive force. The city also alleged that the officer had grabbed the woman by the throat and addressed her with an abusive slur. Despite the incident, and despite a previous suspension for a use-of-force violation, an arbitrator overturned the officer's firing.

August: A Mahnomen County Sheriff's deputy had pleaded guilty to drunken driving in 2015 — his fourth DUI conviction in 17 years. And it turned out he had not disclosed one of his three old offenses when he was hired. He was ordered by the court to install an ignition-interlock system in his car — a device requiring him to take an alcohol breath test every time he drives.

Noting the remote county's difficulties in hiring reliable officers, an arbitrator overruled the deputy's firing, ordering the county to install an interlock breath-test device in the deputy's squad car.

These overruled management determinations — that the cops involved had shown they can no longer be trusted to wield the awesome authority of peace officers — can be questioned, of course. But it's hard to see that they are so insupportable as to deserve no deference. Not, at least, if the public interest and public safety come first.

It is easy for politicians, activists and pundits to demand justice and call for a change in police culture or even for a transformation of society. But to take on the politically powerful police unions — to insist on some kind of change in disciplinary processes that would empower law enforcement officials to weed out cops who shouldn't be cops — well, that would take practical courage and clarity of thought.

Rather like good police work itself — but a lot more rare.

D.J. Tice is at Doug.Tice@startribune.com.