When an American dentist named Walter Palmer killed a beloved lion named Cecil, the social media platforms that allowed outraged web users to spread the story also enabled them to do more than just fume. It gave them the power to act on their anger, to reach into Palmer's life and punish him for what he'd done, without having to wait for the wheels of more formal justice to turn.

Web users uncovered Palmer's personal information, including about his family, and published it online. They went after his business, a private dental practice, posting thousands of negative reviews on Yelp and other sites. The practice has since shut down. Users also went after professional websites that host his profile, leading the sites to remove his information. On Twitter and on his practice's public Facebook page, people made threats of physical violence.

This should look familiar: It is the same set of tactics that has been used in online harassment campaigns such as the "Gamergate" movement that targeted women in technology, or the seemingly endless online harassment conducted against female journalists. It is a growing trend of internet mob justice, one that often bleeds into real-world harassment with real-world consequences.

We as a society deemed campaigns such as Gamergate unacceptable and rejected their proponents as harassers who crossed the line. But because we all agree that we dislike Palmer, the campaign against him has so far been deemed acceptable, even funny or laudable.

That worries me. And it should worry you. Gamergate has been run out of polite internet society, but the mob campaign against Palmer suggests that the tactics Gamergate employed are not disappearing — they are becoming more mainstream. That should be deeply worrying to us all.

What Palmer did was wrong, and he deserves to be punished to the full extent of the law. But it's easy to forget just how dangerous and unjust "mob justice" is while it's targeting someone you despise. The more this behavior is normalized, the more likely it is to be deployed against targets who might not necessarily deserve to have their lives destroyed — including, perhaps one day, against you.

The campaign against Palmer has crossed the line

At some point on Wednesday, someone taped a sign across the front door of Palmer's now-closed dental practice: "ROT IN HELL."

The sign on the closed office was one of many indicators that the online campaign against him has extended beyond a way for people to express their outrage and is now a way for people to punish Palmer directly.

In the mold of Gamergate, the campaign has targeted Palmer in two ways: by going after his livelihood and by seeking to inflict psychological suffering in the form of harassment and threats. When Gawker Media ran articles that Gamergate disagreed with, for example, the group responded by going after Gawker's advertisers to scare them into pulling their ads. The idea was to threaten Gawker's ability to pay its employees, and thus threaten the livelihoods of those employees. Gamergate also, famously, blanketed its targets, mostly women, with anonymous threats of physical violence.

This campaign against Palmer has been disturbingly successful. His dental practice is closed at the moment, and his harassers are gleeful that they are denying him an income. But this also inflicts harm on people who did not kill Cecil the lion. Palmer's family presumably relies on his income. So do his employees, whose livelihoods are now threatened as well. When a Reddit user pointed this out, over 1,500 users voted in support of the response that "His employees are better off working elsewhere." The mob, naturally, has shown no intention of helping to find new jobs for the innocent dental employees it is seeking to put out of work.

As for the threats, just as in all cases of mass online harassment, many of them were clearly idle, but some were awfully specific. While they will probably come to nothing, the intent of these threats is often not to warn of coming violence but rather to create just enough uncertainty in the target's mind as to the gravity of the threats that he or she will have to worry. That fear is its own kind of psychological suffering, and it forces people to change their behavior in ways that can be socially and economically damaging.

Maybe you loved Cecil the lion, and believe that Palmer deserves all of this suffering. Maybe you believe that his family and employees also deserve to have their livelihoods threatened. But even if you believe that this particular mob made the correct decision in both identifying the targets and meting out punishments, the way its members reached these decisions — arbitrarily, based on what they thought would feel good to punish — should worry you.

Mob justice is not justice

One of the reasons we have a justice system is to punish criminals for wrongdoing, both to serve the abstract ideal of justice and to deter future criminal acts. The idea is that if you are considering committing a crime, you will understand that the justice system has a high probability of catching you and enacting a certain punishment.

Mob justice does not accomplish this, and often does the opposite. One of the biggest reasons is that it is applied extremely inconsistently, even randomly.

The internet mob just so happened to avenge this particular case of a hunter seemingly illegally killing a rare wild animal, but not other cases. Defenders of the anti-Palmer campaign argue that this is irrelevant: Palmer did something wrong and deserves to be punished regardless of whether other people also did something wrong.

But that misunderstands how justice, mob or formal, is supposed to work. It treats justice as a sort of random lightning bolt from the sky; one is reminded of the vengeful but arbitrary gods of Greek or Roman lore. If you are a would-be big-game poacher, the lesson you might draw is not that poaching will be punished, but rather that poaching is permissible as long as it only kills animals that are not likely to become beloved on social media. Indeed, that seems to be the lesson that Palmer himself has drawn from the backlash: In a recent statement he apologized for killing a lion "who was a known, local favorite," not for hunting lions in general.

A formal justice system, at least in theory, determines the severity of a crime based on objective factors such as its impact on society and how it compares with other crimes. The internet mob determines the severity of a crime based on subjective factors, such as how unlikable they find the alleged criminal to be, how likable they find the victim, and the degree to which the alleged crime fits into their preconceived beliefs. You'll notice that most of these trace back not to the crime's impact on society, but rather the degree to which punishing the crime will feel good for the punishers.

Jon Ronson, in a recent New York Times Magazine piece, explored how this culture of mob justice emerged out of more innocuous practices of mocking people who'd said or written something dumb, and how it became more about serving the mob than about serving justice:

Still, in those early days, the collective fury felt righteous, powerful and effective. It felt as if hierarchies were being dismantled, as if justice were being democratized. As time passed, though, I watched these shame campaigns multiply, to the point that they targeted not just powerful institutions and public figures but really anyone perceived to have done something offensive. I also began to marvel at the disconnect between the severity of the crime and the gleeful savagery of the punishment. It almost felt as if shamings were now happening for their own sake, as if they were following a script.

This gets to one of the root problems with mob justice: It is not primarily about punishing the crime or the criminal, but rather about indulging the outrage of the mob and its thirst for vengeance. Sometimes that leads the mob to target people who perhaps legitimately deserve punishment, but typically it does not. And there is no reason to expect it to. That's not what mobs are about.

The formal justice system derives it decision-making from written laws and generations of precedent; it is adjudicated in a highly formal and regulated environment. It is often flawed, but is at least designed with the goal of fair and consistent treatment for both the accused and the accuser.

Mob justice, meanwhile, is derived from the collective feelings of whoever happens to be participating. The mob's case law is limited to whatever its participants happen to remember and care about in that moment. Its rules of evidence privilege anything that shares easily on social media and that confirms the preexisting belief system of the mob participants. That is a way of administering justice that is just as likely to target innocent people as guilty ones — especially because there's no definition of what "guilty" and "innocent" mean in the first place.

The proliferation of mob justice should worry everyone

Because mob justice does not work by any consistent system of determining guilt or assigning punishment, in theory anyone can be targeted. Of course, the formal justice system can make mistakes as well. Often, those mistakes are rooted in systemic inequities such as disproportionate sentencing for people of color. But at least the formal justice system is designed to give people the means to redress those mistakes and to reform systemic issues.

The fact that mob justice lacks these mechanisms is not a coincidence. It is built into the very practice, because the mob is not about justice in the abstract sense of furthering society's collective good. Rather, it is about pursuing vendettas — for example, Gamergate's fury at the growing role of women in technology, or Reddit's open hatred of people who are overweight — or about simply indulging the mob's desire for blood.

If the mob just so happens to pick a target that we find despicable, such as Palmer, that does not make their actions "justice." Condoning it as such, or even shrugging it off as not such a big deal, allows these behaviors to take root. That has more significant consequences than just convincing more people on the internet that online harassment campaigns are acceptable. These campaigns build on one another as they develop systems of organizing. Their participants have learned to leverage mass community sites like Reddit and 4chan as platforms to launch and fuel mob campaigns, and to learn and disseminate effective harassment tactics. If online harassment is a skill, there are now more and more places to study it.

That puts all of us in danger. The mob is free to decide on its own what does and does not constitute a crime — being a woman who works in technology or journalism, for example, or having the wrong religion —and any one of us could someday be implicated in such an "offense." And even if the mob targets acts that are indeed criminal, it is all too easy for them to get it wrong.

In April 2013, shortly after the Boston Marathon Bombing, users on Reddit came to believe that a young man named Sunil Tripathi had conducted the attack. Someone who resembled Tripathi had been visible in photos of the marathon, and as it turned out Tripathi was suspiciously missing. The internet mob turned on Tripathi's family, bombarding it with hundreds of threats; his sister received 58 phone calls just between 3 and 4 am that night.

A few weeks later, the Tripathis, after enduring the trauma of online and real-world harassment, learned that Sunil had in fact died in an apparent suicide some time before the attack. Reddit users apologized, but it was too late. Mob justice had been done.