euthanasia vs mercy killing

euthanasia and mercy killing both are nouns.

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

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

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

euthanasia (noun)	mercy killing (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)