Personally, I would not attempt to detain him at the store with my firearm. As a civilian, I feel the only time I should draw my gun is if I'm in a life-threatening situation. That clearly wasn't the case. I would have followed the guy to see where he was going just like you did, and called the cops. Would try to avoid any encounter until the cops show up. I don't trust the judicial system. Too bad the bad guys got away with one.
It would have been an interesting situation if you were packing and shot him after he came out with those weapons. Could be interpreted as you are the aggressor, considering you went to his house, etc. Would be watching you on next Saturdays "48 Hours Mystery". Take care.