No such thing as bad dogs - just bad owners.
You can't keep dogs on leads, they need to be able to run and get exercise, just like the rest of us. That doesn't mean that certain breeds of dogs, or aggresive individuals of other breeds shouldn't be walked outside of 'normal hours' (ie early mornings/late nights), or with muzzles, but in the main, if well looked after, even pitt bulls and rotties make good pets.
Our dog (a soppy chocolate lab) wouldn't harm a fly, but I still wouldn't let him run around in a childrens play area. Sometimes accidents happen, and a playful nip can be a serious problem. Rolo has bitten me when playing tug of war. Completely accidental, but if that were a child, he wouldn't be given the chance to argue. We've spent many hours training him though, and he always comes back when called. Even to the point our neighbours sometimes take him out with their dogs so if they call Rolo, their dogs will follow him back! ;D
