At 3:43 the newsman stated that the officer's report stated that the growling dog was showing it's teeth was going up the hill towards him. That's not shown on the video, as the officers gun is blocking much of the view.
How fast can a charging pit bull move that is 20 or less feet away from it's target? It'll be on it's target within seconds.
If the dog was aggressive and posed a threat, then it was justified.
But the fact that he was calling to them instead of waiting for animal control makes that look suspect. Its not like no cop has ever stated something not entirely accurate to cover himself.