Mercy killing (not assisted suicide) and the ramifications of the karmic action

Killing is no way to alleviate a being's suffering.

It's like investing in a degenerate gambler's schemes out of mercy, it ruins both.

Being's suffering is intended and is brought about by bad behavior due to ignorance. Killing is such bad behavior and the being you kill can not escape his bad kamma.

If you do this kind of killing then you are fit to be some torturer in hell, someone who kills beings who appear in hell as to liberate them from living there.

The Pali suttas say killing without mercy is hellish, as follows:

Bhikkhus, possessing.... qualities, one is deposited in hell as if brought there. What...?

Here, someone destroys life; he is murderous, bloody-handed, given to blows and violence, merciless to living beings.

AN 10.211

The same sutta says having mercy leads to laying down weapons, as follows:

Someone with... qualities is raised up to heaven. What...? It’s when a certain person gives up killing living creatures. They renounce the rod and the sword. They’re scrupulous and kind, living full of compassion for all living beings

The Pali suttas also say what is unwholesome is rooted in greed, hate & delusion, as follows:

And what, friends, is the unwholesome, what is the root of the unwholesome, what is the wholesome, what is the root of the wholesome? Killing living beings is unwholesome; taking what is not given is unwholesome; misconduct in sensual pleasures is unwholesome; false speech is unwholesome; malicious speech is unwholesome; harsh speech is unwholesome; gossip is unwholesome; covetousness is unwholesome; ill will is unwholesome; wrong view is unwholesome. This is called the unwholesome.

And what is the root of the unwholesome? Greed is a root of the unwholesome; hate is a root of the unwholesome; delusion is a root of the unwholesome. This is called the root of the unwholesome.

MN 9

AN 6.63 says kamma is intention, as follows:

Intention, I tell you, is kamma. Intending, one does kamma by way of body, speech and intellect.

And what is the cause of kamma? Contact is the cause of kamma.

AN 6.63

There two or three suttas, such as MN 144, where a monk in the Buddha's time ends their own life using a knife due to unbearable totally incapacitating pain from some terminal illness. In these suttas, other monks do not help them end their life. This said, monks are totally forbidden by Vinaya from helping or encouraging another person end their life. A monk that even encourages another human being to end their life is immediately defeated and is no longer a monk.

Therefore, the kamma of mercy killing, that is, ending the life of a creature that will inevitably & imminently pass away is obviously not as unwholesome as killing due with violence due to hate or greed. However, the question must be asked whose pain is being alleviated? Is it the pain of the injured/dying or the pain of oneself? Also, the question must be asked: "Is there aversion within myself to to dying creature?"

Veterinarians often put down animals. Possibly you can research and read about their experiences, such as here.

Ultimately, Buddhism explains in its 1st Noble Truth suffering is attachment to the five aggregates (rather than physical pain). Unless we have some special psychic power, we cannot know if & how much attachment occurs in the mind of various animals.

For example, on one occasion, I came across a wallaby (small kangaroo) with back legs broken and disabled, recently struck by a motor vehicle. The wallaby did not appear to have much or any attachment. The wallaby appeared to not be asking anything from me. I phoned the local nature-care rescuers but, by the time they arrived, I observed the wallaby's lifeforce end. It was sad but I spent my time letting go of any of my own attachment and reflecting on Dhamma.

A parable I heard recently (somewhere on this site, perhaps..?):

Two monks walking in the woods ran across a deer lying on the ground, pierced by an arrow. They stood there, listening to the labored breathing of the animal, debating whether the cycle of reincarnation would begin at the beast's last breath, when the last light of life left its eye, or at some other significant point. But just then the Buddha walked past, pulled the arrow from the deer's side, and continued on his way.

Now in the parable the deer survives and goes back to life in the forest. But would the parable have a different moral sense if the deer had died instead? The Buddha didn't pull out the arrow to save the deer's life or to kill the deer; the Buddha pulled out the arrow to ease the beast's suffering. Or perhaps (if we want to take the parable a step deeper), the Buddha pulled out the arrow to ease the monks' suffering.

It's worth remembering that 'suffering' is not the same as physical pain. Suffering is a mental phenomenon: a tension between the way the world is and the way the world should be that causes distress. The arrow did not cause the deer suffering. The arrow caused the deer pain, and the deer suffered to the extent that it expected a world without such pain. The arrow did not cause the monks any pain at all, but nonetheless they suffered, caught in the trap of intellect as they tried to work out the deeper meanings of the moment. Only the Buddha was free from suffering: he saw, he responded, he let go.

The difficulty with the idea of mercy killing is that it is too easily tangled in intellect and ego. It smacks of rationalization, of the egoic thought: "I have the power to end this person's suffering by my own conscious act, and thus prove what a good and wise being I am." It's a metaphysically dangerous path to tread. But inaction is also metaphysically dangerous, leading one to the cold, callous view that the suffering of others is a mere intellectual curiosity. It's difficult...

A buddha would do what needs to be done in any given situation, but a buddha wouldn't try to know what to do in every situation. The idea that there is one ironclad moral rule that covers all cases is a Tolkien-esque fantasy/illusion.