Officers rarely face criminal charges after shooting people, a fact that has played into nationwide protests in the past year over how the police use deadly force.
Yet this case took a swift and unusual turn after the video, shot by a bystander, provided authorities with a decisive narrative that differed from Slager's account.
"It wasn't just based on the officers' word anymore," said Chris Stewart, a lawyer for Scott's family. "People were believing this story."
He said the video forced authorities to act quickly and decisively, and he called the person who made the video a hero.
"What happened today doesn't happen all the time," Stewart said. "What if there was no video?"
Scott's mother stood nearby, saying, "Thank you, Lord" and "Hallelujah."
Authorities also pointed to the video yesterday as a turning point in the case. They apologised to Scott's family for the shooting.
"When you're wrong, you're wrong," North Charleston Mayor Keith Summey said. "If you make a bad decision, don't care if you're behind the shield ... you have to live with that decision."
Slager, 33, was fired by the city police department and arrested by the South Carolina Law Enforcement Division, the state agency investigating the shooting. He was booked into the Charleston County jail.
He faces a maximum penalty of death or life in prison.
"It's been a tragic day for many," Eddie Driggers, the North Charleston police chief, said.
The US Justice Department said yesterday the FBI would investigate the shooting, with the department's civil rights division and the South Carolina US Attorney's Office.
The shooting occurred on Sunday (about 9.30am Saturday local time), after Slager stopped a vehicle with a broken tail light. Scott, 50, fled and Slager began chasing him, firing at the suspect with his Taser, according to the police report of the incident as well as city officials.
Footage of the shooting, first obtained by the New York Times and the local Post and Courier newspaper, shows Scott fleeing across a tree-lined patch of grass. From several metres away, Slager then fires a series of shots at Scott, who appears to be unarmed. Scott crumples to the ground.
"Shots fired and the subject is down, he took my Taser," Slager told the dispatcher, according to a portion of the police report filled out by another officer on the scene, relaying what he heard.
Police later said that Scott was hit with the Taser at least once, because part of it was attached to him when other officers arrived.
The video also shows Slager picking up an item and placing it near the fallen Scott, though it is unclear whether it is the Taser. Even if Scott did have control of the Taser, officials said, the video shows that he was too far away to use it against Slager.
"I can tell you that as a result of that video and the bad decision made by our officer, he will be charged with murder," Summey said.
The North Charleston shooting comes after incidents in Ferguson, Missouri, and New York City last year that drew heavy scrutiny to confrontations with police that end with black men dead. Unrest over those types of incidents continued into this year, with a shooting in Madison, Wisconsin, sparking lengthy protests.
North Charleston is the third-largest city in South Carolina, and it has a different demographic breakdown from the rest of the state. While two-thirds of South Carolina residents are white, North Charleston has more black residents (47 per cent) than white residents (41 per cent), according to census figures.
The city's police force does not reflect that breakdown, however. Last year, four out of five North Charleston officers were white, according to the Post and Courier.
The police department announced in February that it would equip the force with 115 body cameras.
City officials stressed that the episode was not indicative of the entire police force of 342 remaining officers, calling it a singular "bad decision" by one officer.
Scott's family praised the decision to charge Slager, saying they were "grateful" to the person who came forward with the video, a lawyer said. The source of the video, a man who appeared to be passing by when the incident suddenly unfolded before him, has not been identified.
Stewart, the lawyer for Scott's family, said the family were "sad" about the shooting.
"There is nothing that can bring their son and brother back," he said, "but they are relieved that charges were filed."
He spoke from Scott's mother's home, where relatives - including Scott's four children and three brothers - had gathered.
Slager, meanwhile, was initially represented by David Aylor, a local lawyer, who said soon after the shooting, "I believe once the community hears all the facts of this shooting, they'll have a better understanding of the circumstances surrounding this investigation."
But yesterday, shortly before Slager's arrest was announced, Aylor told the Washington Post that he was no longer representing the officer.
"I don't have any involvement in that case moving forward," he said.
This was the 11th time an officer has shot someone in South Carolina this year, according to Thom Berry, a spokesman for the state Law Enforcement Division.
City officials and civil rights activists in South Carolina urged calm in the wake of the shooting and the release of the video.
Recent incidents in which black men have died at the hands of US police
Eric Garner
Died on July 17, 2014.
A New York policeman used a banned chokehold technique to restrain the unarmed 43-year-old. He was wrestled to the ground by several officers who claimed he was illegally selling loose cigarettes. Daniel Pantaleo, the only officer investigated by a grand jury, was not charged.
Michael Brown
Died on August 9, 2014.
Brown was an unarmed black teenager shot by Darren Wilson, a white police officer, in Ferguson, Missouri. Some said he had his hands up in the air. In November, a grand jury said the officer should not face criminal charges.
Ezell Ford
Died on August 11, 2014.
Unarmed Ford was shot by two Los Angeles police officers after a struggle on the street. Police said the mentally ill 25-year-old tried "to remove an officer's gun from its holster". In December, LAPD chief Charlie Beck said the investigation was continuing.
Tamir Rice
Shot on November 22, 2014 but died on November 23.
Rice, 12, was shot by Ohio police in a park playing with a BB gun. Police said Rice reached into his waistband for the toy gun when the two officers told him to raise his hands. Responding to the family's lawsuit, Cleveland city said Rice's own actions caused his death, not the police.
