I don't care the size, age, or training. I have a dog and I would not hesitate to put it down if she ever bit anyone.
My grandparents lived on a farm and we always had family reunions there. One year my aunt got bit by their aging dog that my grandma loved. That was the end. My Grandpa handed a gun to my dad and then drove the body out to the forest and buried it. An animal that attacks a human does not deserve a second chance.