“I have something to share with you,” he said. They got into Anthony Scott’s car. Then the stranger showed him the video on his phone. “I knew it! I knew it!” Mr. Scott exclaimed, and what might have been another shooting of an obscure black man by the police suddenly became an astonishing glimpse of life and death, black and white, police and civilian.

Image Anthony Scott, left, whose brother Walter was killed by the police, at his home near North Charleston, S.C., on Wednesday. Credit... Chuck Burton/Associated Press

In the video, a white police officer, Michael T. Slager, was seen shooting and killing Walter L. Scott, 50, as Mr. Scott ran away on Saturday after a traffic stop for a broken taillight. The authorities brought murder charges against Officer Slager on Tuesday, the same day the video was released to the public. And on Wednesday, the city said the officer had been fired, though he will continue to receive health benefits for his pregnant wife until the child is born.

The city’s swift response — which included voluntarily, and almost immediately, turning the investigation of the shooting over to a state agency — underscored not just the power of the video. It also showed the impact of intensified public scrutiny of police violence after police killings of unarmed blacks in Ferguson, Mo.; Staten Island; and Cleveland.

“I have watched the video, and I was sickened by what I saw,” Eddie Driggers, the North Charleston police chief, told reporters Wednesday at an emotional and often chaotic news conference that protesters repeatedly interrupted. Asked whether the proper protocols had been followed, Chief Driggers said, “Obviously not.”

At the news conference, at North Charleston’s City Hall, there were pointed questions about when, and if, first aid had been provided to the victim, and if Officer Slager’s version of events had ever been in doubt before the video emerged. “There are questions that I have in my mind that I can’t answer right now,” Chief Driggers said. He and Mayor R. Keith Summey said they were limited in what they could discuss as a state investigation continued.