Most encounters do not end with violence or death, even if they produce humiliation and tension with police. But they are far more likely to end in a killing in the United States than anywhere else. According to the Bureau of Justice Statistics, police shot and killed 828 people in 2011, a figure that includes all killings, justifiable or not. By comparison, during the same year, there were two fatal police shootings in England and Wales; six in Australia; and six in Germany.

When an incident does turn fatal, the police officer's version of events often proves difficult to contest. An anonymous bystander's video, though, made the Scott case different. Slager's report said that Scott had taken his Taser and that the officer felt threatened. But the video revealed that Scott had been running away from Slager when he fired eight shots into his back. It also showed Slager picking something up and then dropping it near Scott's body—perhaps the Taser. The footage changed it from another disputed tragedy into a murder case.

"I thought that my brother was gunned down like an animal," Anthony Scott, the victim's brother, said. "It was just unbelievable to me to see that."

Yet the family said they thought it was important that the video was shown. So powerful was the video, that when Slager's attorney finally saw it, he immediately dropped him as a client. Video has had an equally potent impact in other cases of police violence—including the shooting of a driver by a South Carolina state trooper last year.

Technology has made it more common for such shootings to be filmed, since many people now carry a video camera in their pocket as a matter of course. A good example came in the Los Angeles Police Department shooting of a homeless man on Skid Row in December, which was captured by both bystanders and surveillance cameras, from several angles. But as my colleague Robinson Meyer writes today, capturing these images also requires impressive courage. Whether officers have reason to shoot or not, the videos prove clarifying, whether they bear out an officer's account or call it into question. Even when police are wearing body cams—as lawmakers are demanding now—third-person footage is a useful and sometimes essential supplement.

In addition to being charged with murder, Slager was fired by the city of North Charleston on Wednesday. If he's convicted, Slager could face the death penalty. That makes this case exceptional. As a grand jury's decision not to charge New York police officer Daniel Pantaleo in the death of Eric Garner vividly demonstrated, officers are seldom charged with crimes when suspects or citizens are killed. A Bowling Green State University report found 41 murder or manslaughter charges for police involved in on-duty shootings over a seven-year period—a stretch during which the FBI counted 2,718 cases of justifiable homicide by law-enforcement officers. Meanwhile, a separate Cato Institution report found that only one in three officers who are charged are convicted.