This kind of difference is not a sign of "psychic deficiency or ethic deformity" like the breed's opponents often say. This is only a result of an ultimate specialization, which caused a  rearrangement of basic instincts that regulate behavior inside of the species. Consider that you don't call a greyhound deficient, because it catches and kills a rabbit and gives it to its master, without eating it; or a collie that guards sheep but also doesn't eat them, like wild dogs do. A behavior useful for the humans is a result of genetic modifications of basic instincts and specialization. A fighting dog is thus not a "degenerate", it is not its fault that it is what it is. 

  A fighting dog is aggressive towards other dogs and friendly to humans. This paradox is also an achievement of long-time selection, anchored genetically. Otherwise it would be impossible for the owner to take care for the dog and train it. It could well be that Staffordshire bull terrier's predecessors have been selected and trained for hundreds of years. The selected dogs were preserved, valued and hardly mated with the next best. The breeders from the past knew nothing of genetics, they had brilliant intuition instead, which allowed them to use objective laws of nature, discovered only in the 20th century, and form the whole variety of domestic animals by this time.

 And now it's high time to deal with the dog fight practice, otherwise out story will be poor. Without feeling any attraction for this kind of shows, we have to consider this historical fact, as Staffordshire bull terrier used to be a professional fighter and owes all its features to generations of gladiator-dogs. People who arrange dog fights professionally (we name them "pit-pros') are organized in associations, of course illegal. What makes a dog invincible, what provides it with the possibility to win against a stronger and bigger rival? Pit-pros call it "gameness". This is a unique feature special for fighting dog breeds and means a physical need to fight to finish in any situation and ability to stay in the ring two and more hours without loosing passion and will-to-win spirit.

To be continued...