mass murder vs carnage

mass murder and carnage both are nouns.

Word	Noun	Adjective	Verb	Adverb
mass murder	Yes	No	No	No
carnage	Yes	No	No	No

As nouns, mass murder and carnage are synonyms defined as:

mass murder and carnage: the savage and excessive killing of many people

Other synonyms of mass murder include butchery, massacre, slaughter.

mass murder (noun)	carnage (noun)
the savage and excessive killing of many people	the savage and excessive killing of many people