Mercy Killing?

According to Buddhism, mercy killing cannot be justified. Mercy and killing can never go together. Some people kill their pets as they cannot bear to see the pets suffer. However, if mercy killing is the correct method to be practiced on pets or other animals, then why are people so reluctant to do the same to their beloved ones? For example, a person who suffer from the last stage of cancer, do the family members have the right to end that person's life, just because of one simple reason, it's to end the person's suffering? No matter how sincere one can be, Mercy killing in Buddhism, is not a correct approach. However, some people try to justify mercy killing with misconception that if the motive or reason is good, then the act itself is good. No doubt their original intention or motive is good. But the evil act of killing which occurs through a later thought, will certainly bring about unwholesome results. On the other hand, a being may suffer owing to one's bad karma, the debt will have to be paid in another existence. As Buddhist, all we can do is only to help to reduce the pain of suffering in others, but not to end one person's life.