Poor breeding, mixed with another breed, improperly raised and/or controlled - all these things contribute to the whole "pitbull" stereotype. NONE of those things have anything to do with breed. Most don't realize the AmStaff, Staffy, and American Pit Bull were bred to have NO human agression and generally do not make good watch dogs. Any dog, ANY, that showed ANY human agression were culled or sold. All human agression was purposely bred out of them for the ring. A few reasons why: They not only had to be handled by the owner, but the caregiver (if there was one), the ref, and the handler of it's competion. Each dog was also washed by the opposite owner to be sure there was no "foul play". Also, in the ring, the dog had to have complete focus on it's competition. Remember, there's a bunch of people and noise. Any distractions due to human involvement would be enough for it to lose a match. All abnormal agression shown by these types of dogs, properly bred ones, are NOT due to it's creation or development as a breed.
The problem is people. The bully breed was just unlucky enough to look the way it does, to be tenacious and brave like any other terrier, and misused and abused like any other thing, living or otherwise by humans.