I mean it's a combination of factors it is never just a simple fact that it is only the breed or the owner. But what I don't get is when people are surprised their dog that is bred and selected to kill things smaller than it, mauls a child, yeah sure you trained it and it's a well behaved dog but it's still the nature of the animal to behave in such manner and I think it makes a bad owner if they deny those facts.
My family has a Jack Russel Terrier, beautiful dog, friendly, loyal, well behaved, social, etc. But if you brought another small animal over, say a cat, lizard, some sort of rodent, he'd go apeshit. It's his nature to fucking kill small animals, his prey-drive would go out of control and he'd be nearly impossible to calm down and control.
On another note I think Bullys are incredibly inbred to the point they have severe problems.