Behavior is influenced by genetics as well as environment. Certain individual animals are more genetically predisposed towards violence than others. Certain breeds of particular species tend to have more of these individuals than others. So, it is possible to have a breed that is violent in that: if you take a random sample of that breed where the individuals are subjected to an identical rearing process more of those individuals will be more violent than average than the average breed has individuals who are more violent than average. (I realize that sentence is probably difficult to digest, but I’m not going to spend 20 more minutes working on this).

Given the data that we have on pit bulls, I think they’re a violent breed. Not all pit bulls are violent, but a pit bull is more likely to be violent than a golden retriever when the two are raised in the same environment.