I have known lovely gentle pit bull types, and evil toy dogs and so much is down to owners not training their dogs properly. However there are breeds with specific bloodlines that have been kept separate for guard dogs and fighting. Dobermans have been separated in this way with the vast majority now as soft as you could wish. The main German line however are bred to work and attack to defend property or persons. I have been bitten by two dogs in my life, a dachshund and a terrier cross that terrorised a whole neighbourhood. I do think as there is such a lot of uncontrolled breeding with dogs traditionally used for fighting a license to keep them should include a requirement to have the dog neutered. Those identified where this hasn't happened should be taken and destroyed. This would mean the loss of these breeds but with no traditional working purpose I think it might be worth doing. I would include pit bulls, and crosses, large type staffies these are the ones usually bred for fighting, a simple weight height check would distinguish between them, and crosses especially with mastiffs or Rottweiler s .