Pets, when behaving instinctively to protect themselves, or their owners, or for food may act violently. These animals should not be killed because of an accidental, chance occurrence.
For example, pit-bulls are stereotyped as aggressive killers because some owners mistreat them, and because they have strong jaws. Some police may respond to distress calls by shooting them. This need not happen. Pit-bulls, like other pets, can react poorly when put in bad situations, but this is the owners fault, and the animal doesn't have to die.