People ask me how I would feel if the cows, chickens, geese, salmon, buffalo, ducks, salamanders, crocodiles, sheep, turkeys, goats, camels and dodo birds were killed painlessly. Would it be okay to eat the animals then?
Pain is irrelevant. There are four strong reasons that taking the life of a sentient being against her will is not justified simply because the method is painless.
First, she is being deprived of her further experiences. I rank this first because there is absolutely no working around it. If someone removes your mother from this world against her will, she will never again experience anything. The timeline is cut and cannot be mended. The presence or absence of pain caused by murder is irrelevant.
Second, killing a nonhuman is the murder of an innocent. This is unjust. The method of murder is irrelevant to result from the crime.
Third, it is a speciesist notion that murdering a chicken is acceptable while doing the same to a human is unacceptable. Modern-day court systems would not permit the murder of red-haired children or black children, because those kinds of appeals are racist, illogical nonsense. Like racism, all defenses of speciesism are faulty, if not absurd.
And forth, it removes her from her family. They absolutely will notice her untimely departure. At the idea of sparing one animal from physical suffering, you create suffering in the members of her family who mourn her. (This point could be argued against by suggesting we engineer single animals with no parents or siblings from test tubes; that we kill all the families and friends together at once; or otherwise engage in sterile yet psychotic behavior. The problem with these ‘solutions’ is obvious: they are speciesist. If these acts were committed against humans, it would be like a scene from a horror movie.)
You already know these answers, of course. Replace the nonhuman in your question with a human. Then, the reasons why we must show compassion are endless.