Here is how Hall described what happened next.
"As the dog came out, he came right at me. It was like a pitbull and it still came as I backed up. Then something happened after that."
Nothing else about the dog's actions is said by Hall in this article.
I guarantee you one thing, if I ever shot a dog who started to lunge at me, I would be making that point loud and clear every chance I got. I wouldn't say "something happened" or that the dog was attempting to move in such a way so that it could attack.
The claim that the video shows a dog either initiating an attack or commiting an attack is once again not backed up by Officer Hall's statements in this article.
http://216.247.183.73/news/0301/10/police_hall.htm
If the only factor was the specific threat posed by the dog I'd tend to agree. However, when providing backup, the imperative is to protect the other officers from any and all threats. It is obvious to me that the specific threat of the dog attacking Hall himself was just a piece of the overall threat which was the disruption of control at the scene.
One could imagine that if the felony stop was legitimate, bad guys could have used the opportunity provided by the disruption to overwhelm a police officer.
Hall's decision to shoot was correct within the context of a legitimate felony stop. How was he supposed to know that it was not legit? The THCP dispatcher had set him up believe that carjacking/robbery suspects were being restrained. The THP troopers were in charge at the scene. He was providing backup. A dog is running into the middle scene and maybe attacking. It looks like a pit bull. A split second to decide.