Hard to say since I wasn't there, but if the crook was herding employees into the back room, that in and of itself, increases the risk factor...
Having worked in LE for many years, I am assuming that this off duty cop saw a threat and reacted appropriately... no shots were fired..if he was on-duty and in uniform, and the suspect was armed with a chain, I am sure he would have taken him off at gunpoint, too.
I have seen some pretty serious head injuries and gashes to arms and torsos from chain fights, so I am hesitant to reduce my threat factor here.
He did not say a Chain Saw but a heavy chain. If he turned his back and opened the register that would have opened a problem. Deadly force would not have been OK'd IMO. If he "aggressed" the Sgt. then he could have been stopped as it would have been considered a deadly weapon