And this is where the definition of the word "reasonable" starts to get tossed around.A gun in a holster is open carry.
A gun in your hand coming through the door is a threat.
IMO, it is not unreasonable to assume that a man walking into an establishment with a gun in his hand is a potential threat. Most "reasonable" people with firearms that I know would have the arm either holstered, holstered and concealed, or secured and concealed in some type of gun case.
Bad move on his part imo...