I think you'll find Dingo that most people on our board subscribe to the idea that bad owners make bad dogs and that there are VERY few bad dogs.
It is my personal experience that pitbulls in general are very friendly with people. I know of quite a few families that have pitbulls and kids and have NO problems.
Is it possible
to have a killer pitbull? Yes. It is also possible a human child to become a sociopath, but it happens RARELY. When it does happen though, it makes the news. Same with Pitbulls.
Is it equally possible to have a killer (add your big dog breed of choice here)? Yes.
Generally when you group obedience train dogs, take them to vets when they need them, give them love, food attention, exercise, you get a good dog.
Neglect the above and your chances GREATLY increase that you get a dog with problems regardless of breed.