mercy killing vs euthanasia

mercy killing and euthanasia both are nouns.

Word	Noun	Adjective	Verb	Adverb
mercy killing	Yes	No	No	No
euthanasia	Yes	No	No	No

As nouns, mercy killing and euthanasia are synonyms defined as:

mercy killing and euthanasia: the act of killing someone painlessly (especially someone suffering from an incurable illness)

mercy killing (noun)	euthanasia (noun)
the act of killing someone painlessly (especially someone suffering from an incurable illness)	the act of killing someone painlessly (especially someone suffering from an incurable illness)