The cases typically come down to the word of officers versus that of inmates, who tend to be viewed as unreliable witnesses. But in the case of Officer Thomas, a lengthy video showed him reacting violently without apparent resistance from Mr. Hyers.

Neither Officer Thomas, nor his lawyer, Joey Jackson, challenged the video’s accuracy. Instead, Mr. Jackson focused on Mr. Hyers’s criminal history; the dangers faced by guards in a maximum- security prison housing murderers, rapists and pedophiles; the unreliability of an inmate witness who kept changing his story; and Officer Thomas’s fear of being spit on.

“Officer Thomas refused to be spit on by a killer,” Mr. Jackson said during his opening statement.

When Mr. Jackson asked Mr. Hyers about the murder that had led to his imprisonment, Mr. Hyers described getting into an argument with his stepfather, and said that “things went a little too far” and that he had shot him three or four times in the head. Mr. Hyers, in handcuffs and leg shackles during his testimony, also said that at one point he had been a member of the Aryan Brotherhood gang.

Mr. Hyers testified that he was in a bad mood on the day of the encounter with the guard, and that during recreation, which Officer Thomas was supervising, he kicked a ball over the fence.

He said that when he and the other prisoners came back inside and lined up against a wall to be searched, Officer Thomas was angry and asked Mr. Hyers whether he had a problem. Mr. Hyers said he never threatened to spit, but at that point, Officer Thomas began punching him.

When Officer Thomas testified, he said that while bending down, frisking Mr. Hyers, “I hear a gurgling sound, he’s looking down.”

Officer Thomas explained that at the prison, if an inmate spits in a guard’s eyes, the guard must take a regimen of pills to prevent infection. “Who wants to be put on a thing like that?” he said. “I’m fearful for my life.”