"One of the officers told me to stop, but it was because I (said) to them that what they did it was an abuse and I witnessed everything," he told CNN's "Anderson Cooper 360˚."
Santana told Cooper an officer told him to wait where he was but eventually he left the scene to go to work.
In other interviews, Santana has said he feared for his life, which almost kept him from revealing the recording.
Santana recalled the moments when he recorded a roughly three-minute video of North Charleston Police officer Michael Slager
shooting Walter Scott
as Scott was running away Saturday. That evidence led to the officer's firing and arrest on a murder charge.
Santana said when he first came on the scene while walking to his job he saw Slager on top of Scott, who was on the ground. He could hear the sound of a Taser in use.
At no time did Santana see Scott go after the Taser. He believes Scott was trying to get away.
"Mr. Scott never tried to fight," Santana told CNN.
Police said Slager used a Taser against Scott, but Slager would also later tell a dispatcher that Scott at some point had grabbed the Taser.
After Slager shot Scott five times, the officer went back to pick up something.
Santana thinks it was the Taser but said he wasn't 100% sure what it was.
Neither the struggle nor the use of a Taser was captured on video, because Santana had yet to begin recording.
The cell phone video, which Santana revealed this week, reignited national outcries surrounding police treatment of African-Americans and led to a murder charge against Slager
, who is white. Scott was black.
Police said Slager pulled the 50-year-old Scott over for a nonworking brake light on Saturday morning. Scott, according to a dash cam video, fled from Slager for unexplained reasons, and the officer chased him on foot.
When Santana's video begins, Scott starts running away from the officer, with Scott's back to Slager. The video shows Slager shooting at Scott eight times before Scott falls down.
'I thought about staying anonymous'
Scott's mother, Judy Scott, told Cooper this week that she would want to thank Santana for coming forward.
"He was there. God planned that. He's the ram in the bush -- I truly believe that," Judy Scott said.
"I think she was right," he said. "God put me there for a reason."
Judy Scott said she couldn't watch the whole recording.
"When I saw my son running and I saw the policeman behind him, I couldn't take it," she said. "I had to turn away. I couldn't handle it.
In an interview with NBC's Matt Lauer, Santana suggested that he was giving media interviews in part to protect himself against retribution.
"At some point I thought about staying anonymous, and don't show my face, don't talk about it. But ... if I wouldn't show my face, everybody over there knows, including the police, who I am," Santana said.
Santana did not say whether he had received threats.
His attorney, Todd Rutherford, said Santana's video would be useful in an investigation -- not only into the shooting but also into whether Scott received prompt medical attention.
In the last half of the video, a second officer appears to examine Scott with gloved hands.
Santana said after he stopped recording, he watched for a few more minutes but never saw anyone perform CPR.
A police report says a third officer -- not shown in the video -- reported seeing an officer administering first aid, and that the third officer approached and helped that person "with first aid and CPR."
On Wednesday, asked whether CPR was performed on Scott, North Charleston police Chief Eddie Driggers said: "In the end of it (the video), what I saw was (what I) believed to be a police officer removing the shirt of the individual and performing some type of life-saving (procedure), but I'm not sure what took place there."