hatchet man vs assistant

hatchet man and assistant both are nouns.

Word	Noun	Adjective	Verb	Adverb
hatchet man	Yes	No	No	No
assistant	Yes	Yes	No	No

As nouns, assistant is a hypernym of hatchet man; that is, assistant is a word with a broader meaning than hatchet man:

hatchet man: one whose job it is to execute unpleasant tasks for a superior
assistant: a person who contributes to the fulfillment of a need or furtherance of an effort or purpose

Other hypernyms of hatchet man include help, helper, supporter.

hatchet man (noun)	assistant (noun)
one whose job it is to execute unpleasant tasks for a superior	a person who contributes to the fulfillment of a need or furtherance of an effort or purpose
a professional killer

hatchet man (adjective)	assistant (adjective)
	of or relating to a person who is subordinate to another