If they are so fearful and frightened about a black man having a gun.
Where are you drawing your conclusion that the officer's fear was tied to Castile's race from? The power of pure, baseless assumption?
You don't think this officer might've behaved exactly the same way if a white man, or a fellow Amerindian, had told him he had a gun and then started reaching around his pocket and ignoring commands not to?
Where was the NRA? Why are they totally silent about a law abiding gun owner shot dead seemingly doing everything right?
The NRA has a lot of police officers as members and all NRA and CCW type culture / classes / seminars / speakers / books tend to be heavy on protocol and emphasizing the right way to behave in an encounter with police while legally armed. I'm not in the NRA and I don't have a CCW, but I have watched some of their material and one of their big names is Massad Ayoob.
Ayoob is a former police officer himself and he has a lot of videos out there, instructional and seminar, where he talks about this stuff. These people are BIG on things like "calmly tell the officer you have a legal firearm and a CCW, keep your hands in plain sight, and ask the officer what he'd like you to do next." etc.
The logical reason why they didn't rush to Castile's defense is that they had police telling them that he did not conduct himself in anything remotely like the ways their entire culture repeatedly emphasizes that you need to. Why would they go to bat for someone who, to a great extent, personifies what NOT to do according to all their pre-existing materials?
Or it could just be that they're all racists!
(I actually agree they're all racists, because this planet is home to about 8 billion racists. I also agree that the 2nd amendment is actually for white people, because this entire country was constructed for white people and ALL legal frameworks were made with them in mind. But that's beside the point.)
The thing that struck me on watching the video is how the mere mention that the driver had a gun threw the cop into a complete panic.
I find it interesting that you'd be struck by something that didn't happen.
Watch the video again. When Castile first says "I have to tell you I have a firearm" the officer's initial reaction is quite calm. He says "okay, well don't reach for it then" and he says this quite calmly. It's 100% clear that he did NOT go into a panic, let alone a complete panic, at the mere mention of the gun. It was what Castile did with his hands, and his heedlessness of repeated commands from the officer, which sent the officer into a panic.