mass murder vs butchery

mass murder and butchery both are nouns.

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

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

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

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

mass murder (noun)	butchery (noun)
the savage and excessive killing of many people	the business of a butcher
	the savage and excessive killing of many people
	a building where animals are butchered