Pitbulls. Staffs, English Bull Terriers etc are all from the same Lines. Bulldog, and Terrier. They CAN become naturally aggressive to Dog's but if brought up around them there is a much less chance of the aggression.
When they were bred any aggressive dogs (TOWARDS HUMANS) were culled so that it didn't carry on throughout the bloodlines. The only reason you read of these aggressive ones is people inbreeding and chavs who lock them outside their backgarden all night and throwing them around to make them aggressive and look hard.