There was a big story in the UK news last week, where a 8 day old baby was killed by the family dog.
It was a Jack Russell.

And I've personally only had experience of one serious dog attack - last summer a couple of dogs that were constantly cooped up, and barked at everything that went by.....
They escaped, and attacked a dog that was playing on the grass with a little kid.
Luckily the kid was untouched, but the kid's pet was almost killed, right in front of her - Blood everywhere.
The dog victim survived, but it was a close thing.
The aggressors were a pair of Border Collies.
The victim was a Staffordshire Bull Terrier.

It's not the breed that is the deciding factor in behaviour - but ownership - upbringing, training, and treatment are BY FAR the biggest factors in whether or not a dog will go bad.