[1] : WordNet (r) 2.0

mercy killing
     n : the act of killing someone painlessly especially someone
         suffering from an incurable illness syn: euthanasia
see also:
euthanasia 

[2] : 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

Mercy Killing >> M Words
Mercy Killing, definition of term: Mercy Killing
mercy+killing_pag1.html

Mercy >> M Words
Mercy, definition of term: Mercy
mercy_pag1.html

Killing >> K Words
Killing, definition of term: Killing
killing_pag1.html

Mercy Seat >> M Words
Mercy Seat, definition of term: Mercy Seat
mercy+seat_pag1.html