The Walter Scott shooting in South Carolina

Walter Scott, a 50-year-old black man, was unarmed, facing away from a white police officer, and haphazardly attempting to flee in North Charleston, South Carolina. That didn’t stop the officer, Michael Slager, from firing his gun at least eight times at the fleeing man, killing him.

Earlier this year, Slager pleaded guilty to a federal civil rights charge related to the shooting, admitting that he did not shoot in self-defense and acknowledging that he knew his use of force was unreasonable. Then, on Thursday, Slager was sentenced to 20 years in prison.

Legal standards generally give officers a lot of legal room to use force without fear of punishment. The intention is to give police officers leeway to make split-second decisions to protect themselves and bystanders. And although critics argue that these legal standards give law enforcement a license to kill innocent or unarmed people, police officers say they are essential to their safety.

But there are other reasons police are rarely charged and convicted, as well. Sometimes the investigations fall onto the same police department the officer is from, which creates major conflicts of interest. Other times the only available evidence comes from eyewitnesses, who may not be as trustworthy in the public eye as a police officer.

Charging a police officer with a crime after the death of a civilian is incredibly rare — even when there’s video evidence. And a big part of that is because of the legal standards for when a cop is allowed to use deadly force: what the public sees as crossing the line may not actually break the law, and even the most reliable video evidence might not show an officer actually committing a crime.

There are plenty of guidelines for use of force by police, but it often boils down to what the officer believed when the force was used (something that is notoriously difficult to standardize), regardless of how much of a threat actually existed. Here’s how prosecutors draw the line between a justifiable use of force by a police officer, and a crime.

Three days after police officer Michael Slager killed Walter Scott, an unarmed black man, during a traffic stop in North Charleston, South Carolina, video of the fatal shooting surfaced.

That video, filmed by a bystander, told a very different story than the one Slager told investigators after the shooting. It did not show Scott trying to grab Slager’s Taser, as Slager alleged. Instead, the shocking footage showed Slager shooting a fleeing Scott in the back, then picking up something — possibly his Taser — and dropping it next to Scott’s body. On Tuesday, Slager was charged with murder.

Ever since Michael Brown’s death at the hands of Ferguson, Missouri, police officer Darren Wilson last summer, “Black Lives Matter” has been the unofficial rallying cry of people who are fed up with racialized police violence and the wildly disparate rates at which African Americans are killed by people charged with enforcing the law. (Read more: This chart explains why black people fear being killed by police)

In some ways, “Black Lives Matter” is — or should be — an obvious statement. After all, few would actually argue that the life of anyone, of any race, is completely worthless. That’s the thinking that inspired the more general “All lives matter” version, which has been embraced by many Americans as an alternative slogan. And it’s true — all lives do matter.

Fox News host Sean Hannity is a vocal defender of police, as he demonstrated when he stood by the officer who killed Michael Brown in Ferguson, Missouri. But he drew the line Thursday, arguing in a testy exchange on his show that there’s no justification for the grisly shooting of Walter Scott.

“You do not shoot an innocent man in the back eight times in cold blood like this,” Hannity said.

Vox’s Anand Katakam created an interactive map with data from Fatal Encounters, a nonprofit trying to build a national database of police killings. It shows some of the deaths by law enforcement since 2000:

A huge majority of the more than 5,600 deaths on the map are from gunshots, which is hardly surprising given that guns are so deadly compared to other tools used by police. There are also a lot of noticeable fatalities from vehicle crashes, stun guns, and asphyxiations. In some cases, people died from stab wounds, medical emergencies, and what’s called “suicide by cop,” when someone commits suicide by baiting a police officer into using deadly force.

The father of Walter Scott, a seemingly unarmed black 50-year-old who was fatally shot by a South Carolina police officer on Saturday, told Today’s Matt Lauer that his son’s death would have been swept under the rug if an anonymous bystander hadn’t recorded the incident and turned over the footage to authorities.

“It would have never come to light. They would have swept it under the rug, like they did with many others,” Walter Scott Sr. said. “When I saw it, I fell to my feet, and my heart was broken. I said, ‘Oh no, it can’t be.’ When I saw it, I just couldn’t take it anymore.”