I think that when we hear about something tragic like this happening, it is easy to quickly draw conclusions.
I really think it is garbage in, garbage out for the most part with pitbulls or any other dog breed. I have seen so many of these dogs that would rather lick a person to death than ever bite. Unfortunately, many people get pitbulls for the sole purpose of trying to have "tough" dogs, and thus create a mean dog by how they raise them. Just by virtue of having the dog for 3 years does not mean a lot. The owners of the dog might have been bad owners for 3 years, which could create a volatile situation.
Undoubtedly there is always a chance that a dog, just like a person, has a screw loose and is aggressive, regardless of breed.
I have a doberman that just turned a year old and weighs right at 100 lbs. We have had the dog since he was 8 weeks old. My intent when purchasing our dog was to have a dog that would protect my 8 year old, and 2 year old. Despite the fact that our doberman is incredibly sweet and gentle with my girls, I would still never leave my girls with him unattended.
I have not trained my dog to be aggressive, as there is no need. His natural instincts to protect my daughters and the family will come out if a stranger ever tried to harm any member of our family...I assure you, but he is about as kid of a dog as you could ever imagine albeit a very menacing looking dog to strangers with his cropped ears and his size.
People who buy dogs that do not know the history of the animal, or do not respect an animal that has an injury are asking for problems, especially if the animals have not been properly socialized with people and other animals.