Most people believe that most cops are good. Almost nowhere can you find public discourse on police misconduct without references to "a few bad apples" or "exceptions to the rule."
When I say there are no good cops, I do not mean that there are no cops who do good (or even libertarian) things. That claim would be easily disproven using examples such as the Texas sheriff who subverts state and federal law by refusing to jail people for marijuana possession or the cops who delivered Christmas gifts instead of tickets. And certainly not all police use unnecessary force. So why do I claim that there can be no such thing as a good cop?
Police agree to perpetuate injustice
A police officer’s job is to enforce the laws, many of which are unjust. To paraphrase Robert Higgs, every police officer, as a condition of employment, must voluntarily agree to enforce unjust laws.
From the libertarian perspective, then, the police department is a criminal organization. Imagine for a moment a kidnapping ring. Would we argue whether one can be a "good" agent of a kidnapping ring?
But, you might say, a kidnapping ring is not the same as a police department. While the police do make a habit of abducting people (every arrest, after all, is an act of coercion), they also stop murderers and thieves on occasion. So let's say we have a kidnapping ring that also provides some services to the community. Does that exonerate those working with the organization? No. The Mafia also provides community services, and few outside the organization would argue that it is "good."
Police keep you from protecting yourself
What's the distinction between a kidnapping ring and the police, or between the police and the Mafia? The key distinction is that the police have a "legal" monopoly. Providing a competing service is illegal, and the police — the beneficiaries of the monopoly privilege — are the enforcers. They forestall competition at gunpoint, like the Mob, but under color of law. Take Ferguson, for example, where police stopped Oath Keepers from protecting private businesses from looters, even though the police were unable to do the job themselves.
If you reject the Mafia, then you should reject the police, just as you (presumably) reject other criminal organizations that, as part of their core mission, violate people’s rights. You didn't vote for the police. They are not accountable to you in any meaningful way. You are not in any way morally obliged to consent to their jurisdiction over you. They are just one more gang, but with government approval.
Two objections: societal benefit and good intentions
What if, despite these things, the police have a "good" effect on society? What if the good outweighs the bad?
What is a "good effect"? What is "society"? And a good effect on society compared to what: a simple absence of police, or a robust market system of law enforcement and security?
More problematic from a moral perspective, this line of thought attempts to provide ex post justification for the morality of police action, when moral justification can only be based on an actor’s knowledge at the time he or she acts.
Imagine a mugger who steals from a young man in a dark alley. Because the young man loses his money, he then lacks the funds to buy a gun. He would have used that gun to murder someone, and he is unable to procure a gun by other means.
If the murder is prevented, does the mugger's choice to steal somehow become "good"? I don't know many people who would pardon a criminal on the off chance that his or her actions had somehow been "good for society" in this roundabout way. Many terrible events have had unintended, good side effects, but rarely would anyone argue that those events should have been incited to bring those side effects about. Even Keynesians wouldn't be found saying that economic stimulus effects of the Black Death made it a good thing.
Even if the actions of police officers are somehow good for society overall, they should not, individually or corporately, be seen as "good."
But what if police officers have genuinely good intentions? What if, when locking people away for things that do not violate others’ rights, the officers believe strongly that they are doing what is morally right?
We can dismiss this justification, too, with a thought experiment. What if (as has sometimes been the case) a serial killer believes himself to be doing what is right, due to his delusions about reality? Would this belief make his murderous behavior any less bad? No one could seriously entertain such a proposition. We must dispose of the idea that good intentions can magically make the road to hell "good."
By exiting organized crime and moving to private industry, former police officers can become good, or at least neutral. Some, like Justin Hanners, can even do a good deed on the way out by becoming whistleblowers.
It's not the apples — it's the barrel
This argument over whether police are “good” is not about the actions of individual cops; anyone can argue about the details of 900,000 individuals doing their jobs, some with the best of intentions. It's about a system — a system cops voluntarily participate in — that both perpetrates and perpetuates injustice, and one that violently precludes competitors who would both respect and protect individual rights.