They have turned to euthanasia primarily for aggressive animals and those too sick or injured to be adopted. (articles.orlandosentinel.com)
Though these criteria differ from shelter to shelter, the good ones are designed to make the best pets available, and to save the public from aggressive animals. (poochprofessor.com)
In addition, the bitch is usually the more aggressive animal if left on her home turf. (breedingbusiness.com)