Thats really terrible, but unfortunately nothing new, this is why its so very important to make sure that your pet goes to a responsible home, ask lots of questions,do home checks etc, you can't just accept people are who and what they say on face value.
People taking dogs for fighting are known to pose as ordinary people looking for a pet, men and women too, I am not suggesting that the previous owners of this dog didn't home check or that it was their fault at all, for all I know they did everything in their power to ensure the dog had a good home... but it certainly is a very potent cautionary tale.
R.I.P where no one or nothing can hurt you again... poor dog