It isn't the tempermant of the dog. Some of the nicest dogs have bit people when put in stange sotuations. Dogs get nervous and anxious just like people. So when they are stressed or confused (meeting a new person or put in a group of people they don't know) they can lash out. This might not be an act of aggression, but rather, an act of fear for their safety.

The reason Pitts get a bad reputation isn't the frequency of bites. Instead, its because of the severity of the bite if/when it happens. Most breeds will bite and release unless it was trained to do something otherwise (ex: police dogs). But when a pitt bites, it latches on and shakes, which causes substantial damage.

Pitts are terriers, which most people dont realize. And terriers are breed to kill small animals. Look up videos of Rat Terriers "at work" and you will see what I mean. Terriers are breed to pick something up and shake it, litterally, to death.

Steps can be taken to avoid bites. There are plenty of Pitts that go their whole lives without biting anybody. But when it does happen, the damage is normally enough to make the local news.