The laws on this are different in each state, so you should probably post it in the proper state section.
I walk my dog, german shepherd/rottweiler mix, at night and I carry my pistol just in case of another dog attacking us. I feel that if the dog presents a threat to you or your dog, you shouldn't have any problems if you shoot the aggressive dog. I wouldn't hesitate for a second if it was the choice between my dog or their dog. If they can't train them or control them, then I will end the threat to my animal.