You have to remember that a lot of pit bulls that become pets (I'm talking anything under the general label of pit including mixes and purebreeds alike) are NOT bred to fight.
Bottom line, any dog will bite when provoked. Any dog, down to the smallest chihuaha can do a LOT of damage when attacking. I've seen small dogs do MUCH more damage than larger dogs.
Ideally, no animal should ever be trusted 100%. Some animals can be and may be very close to 100% trustworthy around other people, animals of their own kind, and animals of other species, but others can not. Many, if not most dog, horse, bird, cat, etc attacks have been due solely to carelessness...either on the part of the owner or the victim of an attack.
Barring medical issues, I've come to learn that most animals will not just suddenly attack. There are always signs, whether you acknowledge them or not. My dog growls around children. We recognized that right away and no longer allow him to be around children. He is locked away safely whenever there are children present at our house. This DOES NOT make him a bad dog. We recognized a problem and we acted on it. Unfortunately, not everyone is as responsible as we are. This is why people are attacked by animals.
One very similar example has to do with fanatics in religion. There are a lot of insane fanatics in islam, christianity, judaism, etc. Look at how americans treated muslims after the 911 attacks. The fact that several people who referred to themselves as "muslims" crashed a plane into our twin towers did not make every muslim a bad person. People tell me all the time "Oh, that person is not a real christian and does not represent our religion at all". Its true....they don't, and groups of people...or animals...should not be judged on the actions of a small percentage.