I'm not saying that I agree with the officer's decision to fire, because I don't. However, from a strictly legal angle a DA could justify that shooting as aggravated assault by vehicle against an officer. A lethal force response is legally allowed.

Personally, in an investigative situation like that where all the facts aren't yet known, I don't think firing a weapon is a good idea. Yes, the driver was being belligerent and not complying, but the police already know who he is, they have his license plate and they know the exactly vehicle. They could go back to a higher and get an arrest warrant for impeding an investigation, evading, and whatever other charges and then find the guy at home or at work another day. It's never a loss when everyone gets to go home and no one is dead. The jail will still be there tomorrow, and tomorrow brings another chance at arrest.