Obviously the person was a "brainwashed" moron who probably truly believes that all guns are evil as well as anyone who even owns a gun is evil.
You just happened to cross his path that day.
There is no way you will persuade him to change his mind so just blow it off as another citizen who believes what they want to believe, no matter how legal you actually were!
They're out there and wandering around in a daze 24/7, completely oblivious to the real world.
I haven't run into one that was as bad as you metioned, but I've had one or two mention that they would "call the police" if anything happened (whatever that means).
Evidently I had a look that said I was about to go ballistic after finishing my cheeseburger. Who knows!!