Like any larger dog, it’s just a matter of how they’re treated and trained. Any big dog can be dangerous to humans, but I can’t think of any large dog breed that’s vicious by nature. (Though I have to wonder about some of the little dogs! Some of them seem kinda psycho to me.)
Pit bulls can be totally loving and friendly, but if you mistreat it, or train it to be mean-natured, they do get pretty nasty.
Of course I’ve never owned one myself, that’s just what I’ve seen from the dogs I’ve encountered.