it is a mistake to kill or take away someone elses life, its unchristian to do such act, however if you be in the shoes of the person in that death bed you will surely say yes to mercy killing.

the patient or the relative or the person to be called upon emergency has the final say if the patient would have a will of DNR(do not ressucitate), it would mean that if his heart failed to pump, no ressucitate should be perform to revive the patient