He had a holstered gun underneath his vest, Dolloff was reaching for it, that’s why he slapped Dolloff, to stop him from taking his concealed handgun.
If Dolloff knew that Keitner had a concealed gun, his reaching in an attempt to grab it might have been an attempt to get Keitner to reach for it, with Dolloff prepared to beat him to the draw.
But Keitner backed off instead and held on to his spray can.
The missing photos might show whether Dolloff pulled his gun out and aimed it at Keitner before or after Keitner sprayed.