hired gun vs manslayer

hired gun and manslayer both are nouns.

Word	Noun	Adjective	Verb	Adverb
hired gun	Yes	No	No	No
manslayer	Yes	No	No	No

As nouns, manslayer is a hypernym of hired gun; that is, manslayer is a word with a broader meaning than hired gun:

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

Other hypernyms of hired gun include liquidator, murderer.

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