gun for hire vs murderer

gun for hire and murderer both are nouns.

Word	Noun	Adjective	Verb	Adverb
gun for hire	Yes	No	No	No
murderer	Yes	No	No	No

As nouns, murderer is a hypernym of gun for hire; that is, murderer is a word with a broader meaning than gun for hire:

gun for hire: a professional killer who uses a gun
murderer: a criminal who commits homicide (who performs the unlawful premeditated killing of another human being)

Other hypernyms of gun for hire include liquidator, manslayer.

gun for hire (noun)	murderer (noun)
a professional killer who uses a gun	a criminal who commits homicide (who performs the unlawful premeditated killing of another human being)