There are breeds that were breed specifically to be aggressive. Breeds were/are isolated for characteristics in order to fill specific roles, including being guard animals, military tools, etc. Does that mean every animal in that breed will be uncontrollably violent? No, but they do require a handler who knows how to temper and direct the natural aggression/other traits. All too commonly those breeds are the "preferred" for the kind of owner who only wants the breed as a symbol of how they portray themselves (rough and tumble badass who takes no shit) and have no clue or desire to handle the breed properly.