Well, he is probably right about one thing. If they has shot the SOB they would probably all be in jail.
No. If the guards gave the gunman, who was aiding and abetting a major felony, armed robbery, a command to drop his weapon, and he refused to, he could be viewed as a deadly threat and shot, at least in the legs/arms.
They could not shoot the unarmed thief except perhaps with a teaser.