ORANGEBURG, S.C. — For much of the week, Richard J. Combs, a former police chief in the small town of Eutawville, sat in silence at his murder trial while jurors heard how he killed an unarmed black man.

Witness after witness talked about the lingering consequences of a traffic stop, and the 2011 morning when Mr. Combs fatally wounded Bernard Bailey. Sitting at one end of the defense table, he sometimes eyed a canister of smokeless tobacco or fiddled with a pen. But he scarcely spoke to anyone, including his own lawyers.

On Friday, though, with his trial nearing its end, Mr. Combs mounted the witness stand in Orangeburg County’s courthouse and told the jury why he had turned his service weapon, a .40-caliber handgun, toward Mr. Bailey and squeezed its trigger three times.

“I was scared,” Mr. Combs said during about 90 minutes of testimony. “I can’t tell you exactly how something like that feels, but I’ve never been that scared in my life.”