Bottomline, He shot him in the back. The threat was abated priot to the murder, No amount of excuses can support the claim that he felt threatened.
I promise you I'd have shot him in the same situation and "threat to others' is sufficient to justify a police shooting.
PS, I promise that, having a very spotty history with them, I'm not used to building alibis for police.
But I wouldn't start a fight over a traffic ticket either.