While it's definitely questionable as to why they decided to take him into custody at all, we don't know the details of what their basis was, and I'm inclined to think they were violating his rights, The man failed to properly secure his dog. The car window was open.
If I had a Rottweiler run at me, I might put it down. He should have rolled the window up when he put the dog in the car so that there was no way it could get out.
It's true, the policeman maybe should have asked the owner's assistance with the dog for a moment instead of shooting it, but in my mind, any dog in violation of a leash law that puts fear into any human at any time on public property is fair game. I feel anyone should be able to shoot any dog they feel is encroaching upon them. No dog owner is absolved from being responsible for their dog, and that is the risk they run when they fail to keep it leashed.
An error does not become truth by reason of multiplied propagation, nor does truth become error because nobody sees it