While breds have general traits bred into them, you also have to remember that dogs, like people, are individuals. Just like there are people that just "up and snap" one day, there are dogs that will do the same. The differance is that when a little dog does it, it does damage but is rarely leathal, while when a big dog does does it, a full grown adult male can be killed in an instant.
In other words, it's a combo of dog breed, dog size, the individual dog and the owner. There are wondeful dogs with bad owners that stay wonderful. There are wondeful owners with seemingly good dogs that go bad one day. There are wondeful owners with wondeful dogs that stay that way forever. It's an almost endless combination.
What should be done? I honestly don't know. It's a shame to denounce a whole breed of dogs, but at the same time, how do you keep these attacks from happening?
BTW, the most severe injury I've ever had personal experiance with? It was when an 8 pound CAT bit her owner after becoming frightend by an electronic toy that went off near the cat. The cat freaked out, bit my friend on the hand and the bite went all the way to the joint of one finger. She was hospitalized for several days, the bite wound became instantly infected and she could have lost her finger!
Both cat and owner are doing fine, now! My friend got rid of the toy and since she's single, doesn't need to worry about the cat being near children. But it just goes to show you, it' doesn't have to be a large dog to pose a threat, any animal can be dangerous!