Being that you didn’t see what he saw, I’m not sure how you can be so certain of the validity of his actions.
This perp had a gun and reached for it, that’s showtime.
There’s a term for this, “the quick and the dead”.
There’s another term for it: “infringement of the right to bear arms”.
Watch the video. The immediate reaction to being informed that he had a firearm was to panic. But it’s beyond ridiculous to think that Castille would have informed the officer of that fact had he any intent to use it. The immediacy of the reaction to the statement makes it clear that it was the very presence of a firearm - the right to bear arms - that triggered the reaction, not anything he saw.
Watch the video. It’s immediate - no pause. The exercise of his right to bear arms was the proximate cause of Castille’s execution.