It's often unclear because the Staffie is classed as a pit bull along with the banned pit bulls, and pit bulls in general come top for attacks and fatalities. But they are definitely one of the most likely breeds to attack and to kill.
dangerous dogs
"The rise in the attacks coincides with the exemption of Staffordshire pit bulls from the Dangerous Dogs Act."
Of course staffies can be lovely and are known for being great with kids. That doesn't change the fact that any dog can attack, for various reasons, and in that situation, a dog with massive powerful jaws, that is trained to hunt, fight and lock on, is going to attack worse and cause the most damage.
These threads always have so many black and white views on them, but it seems obvious to me that it's both. A dog will be much less likely to attack if their well supervised, trained and socialised - of course. But B when that doesn't happen, it is also a breed issue because when they do attack (which staffies sometimes do, sorry but that's a fact) then certain breeds are more dangerous.
The Jack Russell is statistically the most likely breed to bite, but because it's small, it's much, much less likely to end in a fatality than with a staffie.