Put yourselves in the officer's position. An eighty-pound Rottweiler - an incredibly vicious dog - lunges aggressively at you and you have a weapon in your hands. What are you going to do, let the Rottweiler gnash at you? It was in self-defense and it was justified. I'm sure he would have used a more passive method like a tranq dart given the opportunity, but the thing is, he wasn't given the opportunity.