12 ga. with 00 buck would have severely injured or killed some and sent the rest of those perps running for the hills. Problem is, you not only need the weapon but the situational awareness to pick it up and use it. The old fellow without a shirt seemed more like a deer in headlights, simply shocked and stunned by what was happening. A lot of people in his situation would have reacted the same way, hell I'm not sure how fast I would have reacted, it was over in seconds.
After you've intentionally killed people, be it in the military or some other "legal" situation many of us start wondering if it was worth it. For me, it was always an us or them situation, it was never to protect "stuff". I'm not sure "stuff", especially if it's insured, is enough moral justification to kill. I wouldn't have a problem shooting at their legs with a shotgun or aiming to kill if they were coming at me or other innocents with weapons.
Some people will say the old guy should have been wearing a weapon, the problem with that is, in most retail businesses other than gun stores, employees wearing weapons is bad for business. Most retail customers feel intimidated by that.