This reminds me of a high school buddy.
One day he found a grown-up dog in a supermarket parking lot early in the morning, while on his way to school. He called his parents and they took the dog to a veterinarian.
The vet said it was 2-3 years old and from the scars he had been in many fights over at least a couple of years. It was evident that the dog had been fighting because of the bite and scratch scars and marks, and also because the dog had a tattoo. The dog was pretty severely hurt and maybe he had been abandoned because of that. His suggestion was to put the dog to sleep. But my schoolmate's parents disagreed with the vet and decided to keep the dog as a pet, and have him re-trained.
So they got with a police k-9/schutzhund trainer and they worked with the dog. Strange enough, with time and hard work, they succeeded. They lived near Rome, and they renamed the dog Spartacus after the Roman gladiator who set himself free from slavery. And Spartacus in the end made both a good guard dog, and a fine family pet.
I have lost touch with my school buddy, but last time I heard from him a few years back, his parents still owned the dog, it was about 15 years old and still a good dog.
He said they got questioned sometimes when walking the dog, because of the tattoo - as obviously not every dog you see at the park has 'killer' branded on his flank but they just answered every time that it was a rescue dog and they were not sure about his story.
Quite an unbelievable story, considering that most adult fighting dogs are euthanized no questions asked. And all in all, a happy ending for a dog that was already marked for an ugly death in the dog rings, or for euthanasia at best - he found a home and made it to fifteen years old!