Over lunch, I'd read, in another article, that this officer and his partner had been involved in an episode (in 2012, if I'm not mistaken) where he'd been shot at during the stop. Not that it excuses a thing regarding this incident but it may shed some light on the cop's mental state and why he reacted the way he did as the guy reached back into his vehicle for his ID.
Maybe a little PTSD resurfacing in the moment, I don't know? If he'd been dealing with demons from fighting for his life in 2012, he should have sought help and stepped away from a position where he was uncomfortable. Either way, this dude is and should be done for. Glad his shooting was shit too.