If you read in the article, that dog attacked the porcupine when it walked up to it's house. Obviously, the dog was protecting it's territory. I don't believe there is a single one of us humans that would not do the same, given a situation with an intruder. Ask any vet what they believe to be the meanest breed out there, and they won't tell you the pit bull. If I remember right, it was the Chiuaua or however you spell it.
But whether you have seen it or not, every member of the breed has the genetic capacity to replicate what is shown in the picture above.
Any dog, as you have said, can be bred to do that. There are however, many other breeds of dogs that will do that to protect their loved ones and the hand that feeds them. It is not just the pit bulls. As far as the genetic capacity to replicate something like that, just turn on the local news and you are bound to hear of a father, mother, or another human killing a small child or a child being raped by their parents. Ask the parents why, and they will say, "my parents did it to me so I felt it was ok to do". So, we as humans have a little genetic capacity that we should try and work on as well.