What you have to realize is that most cops are never placed in the shoot/don’t shoot situation. In fact, many of them have never taken their gun out of their holster on duty.
So you have a cop with no experience in “I might have to shoot” situations. She is up against a huge guy against whom she stands no chance in a fight. She tries to give him commands. He ignores her and walks back to his car. Now what?
Had he opened the door and gotten into the driver seat, things might have ended differently. But he leaned into the car, apparently to reach for something.
Now, if she were, say, an active duty soldier on combat duty, she might have had the discipline to wait until she saw a weapon before firing. And she might have had the self confidence to believe that, when she saw the weapon, she would be able to shoot and bring him down before he could shoot her.
But to her, this was a physically huge, apparently deranged guy, who had deliberately walked away from her and was now reaching for something, probably a weapon. She had no faith whatever that, if he brought up a firearm, she could bring him down before he shot her.
What would you have done?