On April 2, 2015, an undercover deputy was conducting a sting operation to try to catch Harris illegally selling a gun.
Bates, 73, was parked several blocks away. He told NBC's "Today Show" he had participated in several hundred sting operations but always in a backup role, where he would "clean up" after deputies and take photos and notes after they had made an arrest.
But officials said as deputies rolled up to arrest Harris that day, the suspect bolted and ran toward Bates. A tussle ensued. Bates fired his gun into Harris' back.
'Oh! I shot him! I'm sorry!" Bates said, as captured in a video of the shooting
His defense attorney, Clark Brewster, said experts will testify about the effects of the stress Bates was under as he made the mistake of drawing his gun and not his Taser that day.
Inside the courtroom
Prosecutor Kevin Gray began in a dramatic fashion. Gray repeated Harris' words in the video after Harris sold an undercover officer a gun, then began to run away from police in a hot pursuit.
"He's running, he's running, he's running," the prosecutor quoted the officer in the video, then clapped his hands to make the sound of the gunshot. "From the time Eric Harris climbed into a truck and didn't know deputy Lance Ramsey was an undercover officer. It took 10 minutes. He had no way of knowing he'd be dead," Gray said.
"You're gonna hear this gun was used by Mr. Bates," the prosecutor said, and held up the gun that is part of the evidence. "You'll hear him say Taser" but you will never see that Taser leave his vest."
The defense emphasized the dangers of undercover police operations, arguing Harris' character was "dangerous" and in the situation "everyone was on high alert and filled with stress." Bates' attorney frequently called Harris by his nickname "40," suggesting to the jury Harris could have been a gangster who "made it known he's a Rollin' 90's crip," Brewster said.
Bates was the only person with a Taser, the defense said. Brewster told the jury that both the Taser and the gun were similar in weight, had the same look and feel and similar laser on them. "When he yelled Taser, Taser, Taser. He mistakenly had his gun instead of his Taser," Brewster said.
The first witness to take the stand on Wednesday, was the undercover officer whose hands and voice are on video as he bought the gun from Harris in the undercover sting.
Fallout at the Sheriff's Office
Shortly after the shooting, critics questioned Bates' qualifications as a volunteer deputy -- and wondered whether his close personal friendship with then-Sheriff Stanley Glanz helped get him preferential treatment.
At the time of the shooting, Bates was a CEO of an insurance company who volunteered as a sheriff's deputy.
An internal inquiry by the Tulsa County Sheriff's Office in 2009 found that Bates was shown special treatment and that training policies were violated during his time there.
A grand jury indicted the sheriff in September on two misdemeanor charges, including one saying Glanz "denied lawful requests for the release of internal investigations into his office's Reserve Deputy program."
After almost 30 years as Tulsa County's sheriff, Glanz resigned.