massacre vs mass murder

massacre and mass murder both are nouns.

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

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

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

Other synonyms of massacre include butchery, carnage, slaughter.

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

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