It sucks but I don't think it was right for the owner to (allegedly) do that. I wouldn't harm a dog, no matter how violent, as it's an animal and really doesn't know any better. I'd take it to the authorities and see if they can do anything about a negligent owner.
Exceptions though if a dog is coming after you then yes you are allowed to put it down as your life (human life) is worth more than any animal.