With this important difference, however: Even with the worst teacher, the effects are diffused across many years and many kids, and it’s hard for just one teacher to do that much damage to any given student. A bad cop, on the other hand, can leave his victim dead or permanently damaged, and under the right circumstances one cop’s bad call — or a group of cops’ habitual thuggishness — can be the spark that leaves a city like Baltimore in flames.

Last December, my colleague David Brooks noted that police unions are resisting change on every issue where police reform might be contemplated, from body cameras for officers to reversing the militarization of local law enforcement. But after the untimely death of Freddie Gray, no issue looms larger than the need to discipline, suspend and fire police officers who don’t belong on the streets — and the obstacles their unions put up to that all-too-necessary process.

The cases from all over the country where unions and arbitration boards have reinstated abusive cops make for an extraordinary and depressing litany. Baltimore is no exception. Last fall, The Baltimore Sun reported on the police commissioner’s struggle to negotiate enough authority to quickly remove and punish his own cops, and the union’s resistance to swift action and real oversight persists.

What we know so far about the officer who first pursued Mr. Gray (his history of mental health issues, in particular) suggests that he might have benefited from being eased into a different line of work. This issue is particularly pressing if you believe that some of the aggressive police tactics criticized in the wake of Mr. Gray’s death, and Eric Garner’s in Staten Island — the stress on quality-of-life and “broken windows” policing, the focus on misdemeanors and disorderly conduct — have played a significant role in America’s declining crime rate and our much-safer cities.

Some liberals have decided these tactics haven’t made a difference, or that they aren’t needed anymore. I think this view is naïve, and dangerously so. But to sustain this kind of police work, it’s necessary to restrain the excesses associated with it; to restrain those excesses, it’s necessary to hold cops accountable. And that can only happen if we reckon with the ways in which police unions, no less than other interest groups, can align against the public that their members vow to serve.