WordNet (r) 3.0 (2006):

mercy killing
    n 1: the act of killing someone painlessly (especially someone
         suffering from an incurable illness) [syn: euthanasia,
         mercy killing]

Moby Thesaurus II by Grady Ward, 1.0:

29 Moby Thesaurus words for "mercy killing":
   bane, blood, bloodletting, bloodshed, braining, dealing death,
   destruction, destruction of life, dispatch, euthanasia, execution,
   extermination, flow of blood, gore, immolation, kill, killing,
   lapidation, martyrdom, martyrization, poisoning, ritual killing,
   ritual murder, sacrifice, shooting, slaughter, slaying, stoning,
   taking of life