You can see the store owner first appears to go for his gun, then either looses his balance or goes for the bad guy. When he pulls the gun from his holster it looks like he ejected the magazine while pulling the slide to chamber a round. Because of the video edit we don't see him re-insert the magazine but it looks like he pulls the trigger, then racks the slide to chamber a round before he begins shooting like a madman.
Having never gone through something like this myself, I can't say I'd do any better under the circumstances, except that I always have a round chambered.
I'd say he's lucky he didn't shoot his customer.