If you watch him closely, it's pretty obvious the dog was barking his head off. the guy stretches out to knock, and does his best to stay back from the door.
he's already scared by the time the door opens, and a pit bull (looks like one to me) comes charging out the door straight at him. No owner in sight, no-one trying to pull the dog back.
I would have shot the fucker too. I don't know this dog. I don't know if he's trying to play with me, or protect his territory. The irresponsible owner allowed the animal to charge at me already. Bad pet owners get people killed when their dogs decide it's time to "defend".