I'll just say that I'm sure the terrorist didn't get his arsenal legally in France and their gun laws make the populace totally reliant on police for protection. The reason why gun sales are all time high in the US is that many (but not all) people logically understand that police/military can't be everywhere to protect them and they might need a gun to protect themselves.  From another news article:
Guns and other weapons were found in a truck that plunged into a crowd of people watching a fireworks display in the French resort of Nice killing at least 77, the regional president said. "There were guns in this vehicle and larger weapons, but I can't say more than that because that is the prosecutor's job," said Christian Estrosi, president of the Provence-Alpes-Cote d'Azur region where Nice is located.