There is no such thing as bad dogs there is only bad owners. With almost a decade of experience working as a vet tech, I have seen nice and nasty versions of every breed. You can always tell immediately what kind of dog you're working with just by reading its body language and it teaches you how to handle them. Cats on the other hand are a completely different story, they'll go from fine one second to a "flying little furry little molatov cocktail" the next.