Funny thing. This exact scenario happened to my father-in-law just after his heart bypass surgery. A police officer who lived in the neighborhood kept a rotweiller (-sp) trained to attack pedestrians who walked in front of the house. He would sit on his front porch, drink beer, and watch the commotion. Kind of a sick freak, if you ask me.
Anyway, my father-in-law was out walking, as per instructions from the surgeon, when the rot charged him. He drew a .45 officers' model from his hip, and laid the puppy out. All this to the sounds of applause from the surrounding neighbors and children, as well as the sounds of the cop squealing like a baby!
Unfortunately, you would not be justified in shooting the owner. The owner poses no threat to you. He was, however, justified in shooting the dog. But, if you want to strike out against the owner for being a jerk, you should simply allow the dog to bite you once before killing it. You can then sue in criminal, and tort civil court for attempted manslaughter, gross criminal negligence, assault with deadly force (deadly weapon?), not to mention several hundred thousand dollars in pain and suffering.
Fearing a chest cavity rupture, my father-in-law did not allow the dog to reach him. Moreover, he felt so bad about having to shoot the thing that he offered to cover the vet bills. Go figure.