Avada Kedavra, the killing curse, may have been created in order to give someone the chance of a quick death. Sometimes, when creatures, magical and mundane, are in excruciating pain, their lives are swiftly ended as a mercy. It is possible that the killing curse was created to achieve these ends. It might have even been used on people with terminal illnesses that wished to choose the timing of their own death, as well. A death that prevents endless suffering for anything living, is a justifiable mercy and use of the curse.