[zahlman]

And, in fact, an officer only needs to have a reasonable belief in the moment (considering "the totality of the circumstances" but only up to that point, and in the officer's own point of view) of an impending threat of that level of violence; that belief doesn't need to be correct, doesn't need to stand up to scrutiny in hindsight (or going frame by frame in a video) and doesn't require the gun to have already fired.