butchery vs mass murder

butchery and mass murder both are nouns.

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

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

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

Other synonyms of butchery include carnage, massacre, slaughter.

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