This like other topics is prone to jump to the extremes. Genetics plays no roll what so ever. Genetics means everything and environment has little to do with how the dog behaves. As usual it's a balance of the two.
Aggression is aggression. Responsibility is responsibility.
Most people don't know what a pitbull is let alone what they are bred for or capable of. There is no shortage of stories though and embellishments here and there make for high drama but sadly is taken for truth more often than should be.
A pitbull we bred to be a tough dog with a high tolerance for pain and endurance. Pit fighting was a sport like boxing or Ultimate Fighting, or perhaps a closer approximation would be to cock fighting. Pit fights are still going on in Canada and the US. Underground but widespread and found in most major cities. That they have become a symbol of thug life only makes worse the plight for the animal.
Underneath all the hype and hoopla is still a sensitive animal no different than the pet you have grown to love and hold dear. All dogs share that in common. But like some kids can be lead astray, so to can animals.
When you breed a large, very strong and aggressive animal with others for those traits, they be come stronger in the line. Many problems can arise from line breeding and it takes a responsible and steady hand to keep the problems from ranging out of control. In irresponsible hands, it's a disaster.
Aggression can and is bred into dogs. But that is only part of the equation. A larger part is the knowledge and good sense of the person who's hands a dog falls into. You can take a perfectly nice dog of any breed and turn it into a highly aggressive biter. On the other hand you can take the most aggressive vicious dog and provide an environment void of any opportunity for that to come to grief. That is the possibility not necessarily the natural outcome.
Dogs that were used for pit fighting in the old days would never consider biting a person. It was the norm for the owner of the other dog to wash yours while you washed his before a match to ensure that things were on the up and up. Though experienced handlers, no dog would ever bite or it was removed from the pit. So originally pitbulls were never bred for human aggression.
It's this fear some have of the big bad world about them that drives the uneducated to start messing with something and turning it into a problem. This is what has happened to a great many number of dogs. Pitbulls are ground zero for misinformation, abusive training, out right bad breeding and over blown hysteria.
Dogs don't raise themselves and then keep themselves, people do that to dogs. As it would just be stupid to take a chain saw, show a bunch of 4 year old kids where the throttle is and then leave it running in a room full of the kids, it is with any dog. It's not that the dog is a time bomb, although some are. It's that there is no one there to tell the kids to stop when they are doing something the dog doesn't want them doing. If it's left up to the dog, then they handle things their way. If they have been bred to be strong and aggressive then they will simply do what is within their abilities. That this kills most babies and small children doesn't even enter into their thoughts.
But who didn't know that a dog could be aggressive? Most people get a dog and want something cuddly and fun to have around and most dogs fit that bill. Some people want a tough dog that can stand up to the rigors of a difficult job. While others want something to make them feel better about themselves and see that person with a well trained well bred dog and want that for themselves, but lack the knowledge and ability to bring that about through years of working with the dog. They play rough with the dog and teach it that aggression with people is a fun game. The dog gets big and strong, the person likes that no one will mess with them and their dog, and then the dog gets loose or allowed to be alone with a kid. Then surprise surprise, tragedy and there is a new round of blame the breed.
Most people shouldn't be allowed to handle any animal. Far less still own and handle an animal that can when in the wrong hands cause devastation. For the experienced in handling dogs, it's like listening to someone explaining that they just set the chain saw down for a few moments in the daycare by the matches and gasoline. Just for a moment, and how could it go so wrong?
Some dogs will never ever harm a child or anyone else because the responsible owner will never allow it to come to that. Yeah as a matter of fact you can watch your dog all the time. Yes you can make sure that there is never a possibility that they will get loose and yes it's possible to see that no one can ever get near your pet without your presence. No different than acting in a responsible way with firearms, explosives, vehicles, drugs and alcohol.
The key is responsibility. Most have it but enough don't that it's a problem.