The Rev. Arthur Prioleau rallies in North Charleston, S.C., on Wednesday after a police officer was charged with the murder of a fleeing man. (Randall Hill/Reuters)

THE VIDEO showing former North Charleston, S.C., police officer Michael T. Slager shooting Walter L. Scott dead is sickening. There is a full-color record of Mr. Slager firing eight shots at the back of a fleeing Mr. Scott, then deserting the felled body, possibly to tamper with the crime scene. By local authorities’ own admission, it was the fortuitous act of a bystander with a smartphone that led to murder charges against Mr. Slager. We can’t help but wonder whether, in its absence, the officer would have escaped with impunity.

Once the footage came out, the reaction was quick and appropriate. Local authorities charged Mr. Slager, who faces death or a term of 30 years to life, and fired him. The mayor and the police chief visited the Scott family. The grieving family urged disclosure from the police and calm from others. “I don’t think that all police officers are bad cops,” Mr. Scott’s brother, Anthony, said. “But there are some bad ones out there.” The FBI, meanwhile, opened its own investigation.

Even though the case against Mr. Slager is moving forward quickly, the events in North Charleston underscore that police officers across the country must change the way they operate. One would think that the minority of officers who violate policing norms might be checked by the knowledge that so many potential witnesses to their acts now carry cameras in their pockets.

Instead of relying on bystanders to provide evidence of wrongdoing, however, police departments should accept and accelerate the deployment of body cameras (which the North Charleston mayor has done in the wake of the shooting). Crime video isn’t always perfect. But, as the Slager episode shows, it can prove crucial. Many departments already equip patrol cars with dashboard cameras. The probability that video evidence will be useful only increases with the number of cameras rolling during any given event. An extra camera on an officer’s body can help reduce problems stemming from dash cams’ limited vantage. Episodes involving multiple officers will produce multiple electronic points of view.

If Mr. Slager had been wearing a body camera, it would also be a lot easier to determine what he picked up off the ground after shooting Mr. Scott. The most plausible speculation is that he moved his Taser close to Mr. Scott’s body, which would have bolstered the officer’s claim that the victim tried to take it. But a body camera wouldn’t have just made the investigation easier; it might have also prevented the shooting in the first place. There’s good reason to believe that the presence of body cameras will change behavior even more than the possible presence of bystander cellphone cameras, encouraging police and those interacting with them to de-escalate.

When situations nevertheless turn violent, investigations should be led by independent law enforcement agencies. North Charleston did that when it turned the Slager investigation over to state police, and the inquiry will probably be more credible for it. Complete information about police shootings should be reported to the federal government, so the country can finally have a thorough accounting of how and how often officers use force in the United States.

North Charleston still has far to go, both legally and emotionally. But the community has made a solid start.