Much has to do with the heredity of the dog, as well as the stupidity of the owner. There are such things as bad lines...dogs bred to bite/attack/fight/kill. Specific traits are passed down the line and become hard to "breed out". Unfortunately, there are too many people breeding dogs...dogs that should NEVER be considered for breeding.
I raised and trained Schutzhund. All my dogs were pink papered, imported from Germany, and were trained to either SchII or SchIII level.
I would consider a Pit a good dog if-and only if- the parents were Schutzhund trained to at least a II level. There is no way on this earth that I would ever have a Pit in my home otherwise. I feel the same about Rotties as well. There are just too many stupid dogs that come from stupid owners and breeders.
What really pissses me off is to see some dumb-ass walking his Pit or Rot with a spiked collar, the dog is not neutered, fat, and limping from hip dysplasia. Oh…and the Rotties with skinny heads…you just KNOW those are poorly bred dogs!