A lawyer representing the Harris family says the answer is simple.
Bates paid big money to play a cop in his spare time, attorney Daniel Smolen says, but he didn't have the training to handle the job.
It's a claim that Bates' attorney and the sheriff's office deny, arguing that he was experienced and qualified for the role. His donations of equipment to the Tulsa County Sheriff's Office and his friendship with Sheriff Stanley Glanz, they say, have nothing to do with the April 2 shooting.
But as Bates faces a second-degree manslaughter charge
, analysts say the case raises serious questions about who's policing America's streets, how and why.
How much training did he have?
Investigators have said Bates meant to pull out his Taser but accidentally used his handgun during the undercover weapons sting.
In a video of the shooting, he's heard announcing that he's going to deploy his stun gun, and then apologizing, saying, "I shot him. I'm sorry."
Critics call it a clear case of police brutality and question whether Bates had the know-how to be a deputy.
"It's absolutely mind-boggling that you have a wealthy businessman who's been essentially deputized to go play like he's some outlaw, like he's just cleaning up the streets," Smolen said.
Scott Wood, an attorney who represents Bates, said his client -- who had donated cars and video equipment to the Sheriff's Office -- had undergone all the required training and had participated in more than 100 operations with the office's violent crimes task force.
He'd never been the main deputy in charge of arresting a suspect, Wood said, but was thrust into the situation because Harris ran from deputies, who were trying to arrest him after he sold a gun to an undercover investigator.
Bates worked for the Tulsa Police Department for a year in the 1960s. He'd been a reserve deputy since 2008, with 300 hours of training and 1,100 hours of community policing experience, according to the Sheriff's Office.
He was also a frequent contributor to the Sheriff's Office, including $2,500 to Glanz's re-election.
Tulsa County sheriff's Maj. Shannon Clark denied accusations that Bates had paid to play a cop, describing him as one of many volunteers in the community who have contributed to the agency.
"No matter how you cut it up, Deputy Bates met all the criteria on the Council on Law Enforcement Education and Training to be in the role that he was in," Clark said.
Clark said the agency is now looking into its reserve program but notes that the 130 volunteer deputies have played a crucial role policing fairs, helping out after tornadoes and rescuing people from burning homes.
Allegations of falsified records
New evidence about the training Bates received was revealed this week, when a lawyer for the Harris family released a copy of an internal inquiry by the sheriff's office in 2009.
The memo did not explicitly say that Bates paid his way into the sheriff's ranks, but the investigation did conclude that he received special treatment
The document outlines interviews with several deputies who allege they were pressured by supervisors to falsify training records or provide exceptions to Bates regarding his duties.
According to the document, one deputy said she was told by the chief deputy to certify that Bates had completed the driving requirements, even though she believed Bates had not done the training.
Another deputy, who was charged with Bates' field training, said that he was pressured into certifying the reserve deputy even though he lacked the required training hours.
The memo also mentions instances where Bates was performing actions -- such as traffic stops on his own -- that he was not allowed to do given his status. When other deputies complained about Bates flouting the rules, the top brass at the department responded by asking that Bates be left alone to do what he wanted.
Deputy: I thought he had a gun
Bates' attorney describes the shooting as an "excusable homicide," arguing his client is not guilty of second-degree manslaughter.
"We believe the video itself proves that it was an accident of misfortune that occurred while Deputy Bates was fulfilling his duties as a reserve deputy," Wood said.
In a statement he made to investigators after the shooting, Bates said he'd attended "numerous schools and seminars related to drug investigations and the tactical operations associated with the apprehension of suspects involved in drug trafficking." He also said he'd attended a five-day homicide investigation school in Dallas and had training from Arizona's Maricopa County Sheriff's Office on responding to active shooters.
The gun he used was his personal weapon, he said, adding that he qualified at the range last fall.
In the report, he describes how, from his position on the perimeter, he saw Harris running from officers.
"I noticed that Harris was running in an unusual way because as he ran he repeatedly touched his right hand to his waistband," he wrote. "Based on my past experience, primarily with the task force, and my past training, I believed that Harris might be carrying a gun."
It was the fear that Harris could have a gun, he says later in the report, that made him rush to use his Taser as he saw Harris on the pavement, struggling with deputies as they tried to arrest him.
"My training on ground fighting, even going back to when I was on the Tulsa Police Department, has been that it is one of the most dangerous situations an officer can experience," he wrote.
'Recipe for disaster'
While many departments have volunteer police programs, such positions are generally used for crowd control or less volatile situations, experts said.
The Oklahoma case raises a troubling question, CNN legal analyst Sunny Hostin said.
"Do we want really what are ordinary citizens, with enough money to play to be police officers, policing our streets? This is a very, very dangerous precedent," she said. "And I think it's now time for either the Justice Department, perhaps, or every single police department to review this, the deputy status, because we're going to see more and more of this kind of thing, if it isn't happening more than we even know."
Daniel Bongino, a former Secret Service Agent, said the New York Police Department's auxiliary department is a good example of a program that works.
"You go through a police academy, and you're primarily used in traffic situations, busy shopping areas. They're not armed, they're usually with an armed regular police officer. I think it's a far better model," he told CNN's "AC360." "I think you were almost setting yourself up for failure in this situation in Tulsa, having a 73-year-old man, however good or bad intentioned, in that kind of situation, with a potential gun crime. That was a recipe for disaster."
Look no further than the recent movie "Foxcatcher" for an example of how donations to law enforcement in order to play the role of a deputy are "very concerning," said Phil Stinson, an assistant professor of criminal justice at Bowling Green State University.
John du Pont, the killer depicted in the film
, made significant donations to police, Stinson told "AC360."
"He had given a great deal of money, he'd given cars, given use of a helicopter, actually set up a firing range for a township police department outside of Philadelphia, Pennsylvania," he said.
"So we've actually seen this before, and if you think about the troubles police departments have had with budgets in recent years, it's rather tempting if you're the head of that type of agency to take someone up on this, and give them the action experiences that they're looking for. It's really pay for play."