I think David French had it right. The cop asks him for his license and registration, Castile volunteers that he has a gun — lawfully, per a concealed-carry permit — then the cop tells him not to pull it out. At that moment Castile’s caught between two commands: Reach for your documents but don’t reach for your weapon. He obeyed the first command (or was he reaching to show the cop his permit?), the cop thought he was violating the second, and in a matter of seven seconds he’s emptying his pistol into Castile. Ironically, Castile was trying to put him at ease by telling him up front that there was a weapon in the car, so that he wouldn’t panic if he saw it. If he hadn’t mentioned it, it might not have been noticed and this would have been a routine stop. He was trying to de-escalate a potentially dangerous situation, and got this.
The defense here is that Castile was given an order not to do something that might cause the officer to fear his life was in danger, and although he apparently obeyed it (Castile says, clear as day, “I’m not pulling it out”), the officer might reasonably yet mistakenly have believed that he was about to be shot. Clear judgment is difficult in the heat of the moment, after all. But if Castile had murderous intentions … why would he have told the cop that he had a gun in the car? He would have kept quiet and ambushed him. If he had gotten angry, if he had suddenly produced the gun without first warning the cop that it was there, then you could have understood the officer reacting defensively. But Castile’s calm as can be. He had a right to have his weapon with him. At worst, you would think, the confusion should have resulted in the cop pulling his weapon and aiming it at Castile as he began to reach so that he was prepared to fire if need be. Watch how he freaks out, though, and starts blasting away. How many other cops encounter drivers every day who have lawful weapons on them yet somehow manage to avoid wasting them on sight?
Keep watching after the shooting itself and you’ll see that the cop continues to flip out over the misunderstanding. This wasn’t a cold-blooded matter of refusing to give a black driver the benefit of the doubt and firing at will. This was a very hot-blooded, panicked overreaction.
He was acquitted of manslaughter, somehow. He’s no longer on the force.