I think you have that backwards. It was the 71 year old who confronted the 43 year old and shot him merely for using his cellphone.
They also show Nicole Oulson’s badly injured hand after she was shot trying to deflect the bullet that killed her husband.
Video from the scene appears to show Oulson snatching Reeves’ popcorn off his lap and throwing it at him. Reeves responds instantaneously with a single, fatal gunshot.
I do believe you may be wrong.
You are correct as far you’ve gone. You left out the attack by Oulson. FWIW, never throw popcorn into someone’s face that has just returned to a dark theater, whose eyesight had probably not adjusted to the dark and who had been through years of training to respond to a threat.
It got Oulson killed.