Well, here's my two cents... if you're entering a house alone, and a good-sized dog threatens to attack you while the human owner is there... that is a situation that could actually kill you. If that dog gets up on you and gets your gun hand, the human owner can finish you off incredibly easily if he's hostile. I absolutely believe that the officer pulled his trigger in self-defense.
That said, the officer should never have been in that situation. Ignoring for the moment that he showed up to the wrong house (as that could be the error of the person making the call), you should never respond to a domestic incident alone. As I hear it, the troubled couple often bands together against the officers even if there is a victim... people are funny like that. Therefore, at least two officers should have been dispatched. With two officers there, the situation wouldn't have been nearly as threatening, and the thought "If this dog gets a hold of me, I'm dead" would never have entered his mind.
I once had some officers knocking at my door thanks to my annoying screaming neighbors, and there were four of them responding to my tiny, one-exit apartment. Even were I not a law-abiding citizen, I wasn't going to try a damn thing... their numbers easily kept tensions down.
As importantly, you don't respond to a POSSIBLE incident with gun in hand... or if you do, you certainly don't have it raised and ready to fire. No, you ask politely for permission to enter, you come in with two officers and you let the people there know, indirectly through body language and tone, that you will shoot them if they try anything stupid BUT there is no reason, at all, that things need to become unpleasant. You don't escalate to holding them at gunpoint unless you know they have weapons or you see a dead body or something... a friendly chat should always be your first resort. Incidentally, this approach would have left the owner free to calm his dog, and probably wouldn't have triggered the poor animal's fight or flight response to begin with.
Now, my guess is that the officer KNEW he shouldn't be there alone (not his fault, orders and all that), was feeling vulnerable, and was trying to compensate with an overly aggressive stance. He shouldn't have been put in that situation, and he should have handled it better.
As for actually pulling the trigger, though? At that point, he had no better option, having already helped paint himself into a corner.
