mass murder vs massacre

mass murder and massacre both are nouns.

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

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

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

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

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

mass murder (verb)	massacre (verb)
	kill a large number of people indiscriminately