WRONG
You don't know that he said anything to management.
He got up and left the auditorium and then came back in a little later without management and then continued the argument.
We do know that shortly after leaving the auditorium and returning, he used his gun.
Is that why he left the auditorium -- to get his gun from his car.
That would make this premeditated murder.
But he came back after a few minutes and moments later, the argument between the two men resumed.
Officials said Mr Oulson asked Reeves if he reported him to management for using his phone.
I only know what I read. But if he did go get a gun that will have to be found out at a later date it not here in the text. I assume he always had it.