Retired North Charleston officer Wade Humphries, a former indirect supervisor of Michael Slager, called Slager his "go-to" man in the department and said that Slager was following his training when shooting at the suspect "until the threat cease [d]".
Slager later thanked the Scott family for their forgiveness.
Through tears, the mother spoke as part of a hearing during which a judge will decide how much time Slager spends in prison for the April 2015 killing following a traffic stop.
Slager, 36, who is white, earlier pleaded guilty to a federal civil rights offense against Scott. In it, Scott could be seen running away from the officer, and Slager was shown pulling out his gun and firing five shots into Scott's back.
He also testified that he could see the taser fall behind Michael Slager.
A federal judge Thursday said he would consider the 2015 killing of unarmed Walter Scott by a North Charleston police officer to be a second-degree murder, not the lesser crime of voluntary manslaughter. Scott's sister-in-law and brother also addressed the court.
"The Scott family is pleased", Rodney Scott said.
For three days, attorneys representing the federal government and a former SC officer charged in an unarmed black motorist's shooting death have presented technical testimony to a judge considering how much time Michael Slager should spend in federal prison.
The case made global headlines when a cell phone video of the shooting surfaced, and became a flashpoint in the ongoing debate about the use of force by police. "No matter what sentence I give, neither the Scott family nor the Slager family will think it is right". Prosecutors have supported the murder finding, which would expose Slager to up to life in prison.
"There's nothing in Michael Slager's background, from birth to today, of any racial animus or any harassment of minority members of the community", his attorney Andy Savage said, countering assertions that the officer acted violently because Scott was Black. That's when federal Judge David Norton said it looked like the taser was coming from Slager's right side.
Slager pleaded guilty to the charge earlier this year. Prosecutors think he put it there to bolster his self-defense story.
Judge Norton opened proceedings Thursday and said he believes it was second degree murder and that Slager committed obstruction of justice.
A pre-sentencing report for Slager found that he committed manslaughter and recommended 10 to almost 13 years in prison.
Savage has claimed that state and federal prosecutors teamed up to go after Slager together.
Officers are rarely charged for deadly on-duty shootings, though that number has increased in recent years amid intense scrutiny and protests that have broken out across the country.
That includes use of Slager's stun gun, which the former officer says Walter Scott grabbed and turned on him, causing Slager to fear for his life and shoot in self-defense. But only Slager and former state trooper Sean Groubert, who shot a man as he tried to get his wallet during a seat belt violation check, will have been sent to prison.