slaughter vs mass murder

slaughter and mass murder both are nouns.

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

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

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

Other synonyms of slaughter include butchery, carnage, massacre.

slaughter (noun)	mass murder (noun)
the killing of animals (as for food)	the savage and excessive killing of many people
the savage and excessive killing of many people
a sound defeat

slaughter (verb)	mass murder (verb)
kill a large number of people indiscriminately
kill (animals) usually for food consumption