According to Buddhism mercy killing cannot be justified. Mercy and killing can never go together. Some people kill their pets on the grounds that they do not like to see the pets suffer. However, if mercy killing is the correct method to be practised on pets and other animals, then why are people so reluctant to do the same to their beloved ones?
When some people see their dogs or cats suffer from some skin disease, they arrange to kill those poor animals. They call this action, mercy killing. Actually it is not that they have mercy towards those animals, but they kill them for their own precaution and to get rid of an awful sight. And even if they do have real mercy towards a suffering animal, they still have no right to take away its life. No matter how sincere one may be, mercy killing, is not the correct approach. The consequences of this killing, however, are different from killing with hatred towards the animal. Buddhists have no grounds to say that any kind of killing is justified.
Some people try to justify mercy killing with the misconception that if the motive or reason is good, then the act itself is good. They then claim that by killing their pet, they have the intention to relieve the unhappy animal from its suffering and so the action 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.
Keeping away from mercy killing can become a nuisance to many. Nevertheless, the Buddhist religion cannot justify mercy killing as completely free from bad reaction. However, to kill out of necessity and without any anger or hatred has less bad reaction than to kill out of intense anger or jealousy.
On the other hand, a being (man or animal) may suffer owing to his bad karma. If By mercy killing, we prevent the working out of one’s bad karma, the debt will have to be paid in another existence. As Buddhists, all that we can do is to help to reduce the pain of suffering in others.