Mercy Killing

Here is an excellent example of mercy killing or euthanasia. Although the Amalekite is certainly lying about the circumstances of Saul's death (cp.1Samuel 31:4-6), he is executed by David for killing Saul (v.14-16).  Yet the story perfectly fits modern day scenarios.  Saul was in great pain.  Death was very near whether someone took his life or not.  He even requested to be killed (v.9-10). These are the very arguments used today to justify euthanasia. Yet, David still viewed the taking of his life as murder.  Why do you think that God opposes killing someone even in these dire circumstances?  How should we deal with those who suffer horribly in their final days on earth?