Terminal illness, e.g. cancer or rabies Illness or accident that is not terminal but would cause suffering for the animal to live with, or when the owner cannot afford, or when the owner has a moral objection to the treatment. A hunters coup de grâce. Behavioral problems (usually ones that cannot be corrected) e.g. aggression - Canines that have usually caused grievous bodily harm to either humans or other animals through mauling are usually seized and euthanized ('destroyed' in British legal terms).