Can't believe what I am reading. I am sorry to hear that some of the dogs member's have adopted turned out to be sick or a bad match, that is on the shelter and partly the owner. You have to really understand the dog.
I've had 3 Pitbulls in the past all at the same time. Granted, they were farm dogs they were the best dogs I have had and by far were the easiest to train. I had them herd my chickens and chicks in the Philippines. Keep in mind I did all of this when I was 6 years old.
Absolutely any dog can be aggressive, the nature and degree of aggressiveness depend on a lot of factors both of human and breed. The good thing about the bigger dogs is they give you a warning (if you know how to look for it), little ones just snap at you. A cute little french bulldog can be very vicious. In terms of aggression Pitbulls I can tell you are NOT the most aggressive, you haven't seen a Presa Canario (sweet dog and great breed but if you have a bad one after you... you will lose guaranteed as no break stick will work I just wanted to illustrate the physical superiority of this dog to a pit).
Pitbulls (in general - there are many different types) were once the number 1 most popular dog breed in America. Rappers, Drug Dealers, etc. have turned Pitbulls into fighting machines and "protection dogs" because they recognized the physical power of the dog but more importantly the degree of loyalty they have towards the owner or sticking to a specific goal. Furthermore just the way they are made makes for a very intimidating dog large head which enables more muscle for a bite hold and also the muscular appearance due to it's short hair. Anyway bottom line, it is mostly in the training of the dog that matters.
I worked with a Pitbull recently who was pulled from Animal Control. His back story was he was used as a "guard dog," I suspect dog fighting or some other criminal reason. This dog was absolutely massive and I also want to mention his previous "owners" cut his ears with scissors to get that fierce ear cropped look. He was one frustrated dog as one would imagine and extremely powerful. Everybody approached with caution but we found that the dog enjoyed exercise. Before interacting we would lure (via positive training techniques) to walks and then to the treadmill for extra workouts. In a week he became the biggest marsh mellow ever. Exercise, positive reinforcement, and socialization with cats, objects, humans, and other dogs assisted in this dogs rehabilitation. I approached him with caution at first but took it slow and now he's in a great home. This is just one Pitbull. I picked mine off the streets when I was younger. I also assisted my friend in training hers who she found on a neighbors yard- fantastic dog as well.
Now with that said, although I do have great adoration for the breed with ANY dog you MUST assess the dog first and see what drives him. When adopting a Pitbull I say take extra caution due to the reasons stated above (not knowing the history of the dog, etc.) Also assess the people that have been working with the dog you will know the difference of who actually knows what they are talking about and who doesn't. Ask them what they do for training and rehabilitation and what test have they done in terms of children and other animals.
I've worked with hundreds of different dogs and assisted in puppy mill rescue and have seen results of these breeding hell holes. Do not buy from a Puppy Store or a non certified AKC Breeder. This is for your own safety (in assessing temperament and health and your wallet) and to help stop the overwhelming amount excess dogs we have due to things such as owner surrenders, strays, etc.
Sorry for the long post... had to weigh in my 25 cents lol.
I am going to cite some examples (although popular and probably overused lol):
Sharky the Pitbull
Junior and Daddy (Who can forget this guy)
Gypsy- Pitbull rescued from Dog fighting (WARNING GRAPHIC IMAGES)
K9 Training for Pitbulls