A very loaded question which should spark good debate.
My answer leans toward making the statement that it's not so much the breed that is bad however the propensity of the individual dog that may be the issue. Whether that particular animal was subjected to fear, mistreated, abused or simply not trained properly may be the triggers needed for that individual dog to become aggressive towards other animals and humans. It many similar ways this rings true with people as well. I think 40% genetics and 60%environment (which I'm including all types of human behaviour, human interaction and breeders advocating mixed breeding) to all play factors in the disposition of a dog's behaviour.
I love dogs and have always had a dog as part of our family structure however, even I'm a little wary in the vicinity of a Staffy or Pit Bull. There are some that I've observed to be absolutely aggressive and some I've observed to be docile. Then there are some small lap dogs who you think are cute and docile and become aggressive almost instantly.