mercy kill may be mercy to those bearing the brunt of

  mercy kill may be mercy to those bearing the brunt of attending to the patient

--but not to the patient lined up to get killed.
no one wants to die, especially at the discretion of others,
--even one is if deaf dumb blind handicapped, paralyzed, immobile, suffering the pain.
best is to ''give  care if not cure'' --by social organisations, if not by near and dear, being sure that nature is so merciful  that ends the life, will not keep anyone alive forever.
the best is nature's mercy.
the very fact that the patient is kept alive by nature is itself a sufficient justification  not to kill.
my experience in attending a cancer patient till end , a torture day and night, yet nature is the supreme one to decide, it taking its own time.