Mercy and Killing can never go together.
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 kamma. If By mercy killing, we prevent the working out of one's bad kamma,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.
Killing for Self Protection
The Buddha has advised everyone to abstain from killing. If everybody accepts this advice, human beings would not kill each other. In the case where a person's life is threatened, the Buddha says even then it is not advisable to kill out of self-protection. The weapon for self-protection is loving-kindness. One who practises this kindness very seldom comes across such misfortune. However, man loves his life so much that he is not prepared to surrender himself to others; in actual practice, most people would struggle for self-protection. It is natural and every living being struggles and kills others for self-protection but kammic effect depends on their mental attitude. During the struggle to protect himself, if he happens to kill his opponent although he has no intention to kill, then he is not responsible for that action. On the other hand, if he kills another person under any circumstances with the intention to kill, then he is not free from the kammic reaction; he has to face the consequences. We must remember that killing is killing; when we disapprove of it, we call it 'murder'. When we punish man for murdering, we call it 'capital punishment'. If our own soldiers are killed by an 'enemy' we call it 'slaughter'. However, if we approve a killing, we call it 'war'. But if we remove the emotional content from these words, we can understand that killing is killing.
In recent years many scientists and some religionists have used the expressions like 'humane killing', 'mercy killing', 'gentle killing' and 'painless killing' to justify the ending of a life. They argue that if the victim feels no pain, if the knife is sharp, killing is justified. Buddhism can never accept these arguments because it is not how the killing occurs that is important, but the fact that a life of one being is terminated by another. No one has any right to do that for whatever reason.
by Venerable K. Sri Dhammananda