Raptor is entirely correct. Poor breeding (or breeding for illegal purposes) can certainly produce aggressive dogs. Genuinely aggressive, not just dominant.

However, that does not make the breed itself aggressive or dangerous--only the mis-bred dogs themselves. You can do the same thing with mutts--breed aggressive to aggressive, and get more aggressive. No dog breed is actually bred to be aggressive towards people across the board. Even the guarding breeds are carefully bred to be loving and protective of their family, and aggressive only toward outsiders.

The Fox Farm experiments show that temperament is heritable in canines.