murder mystery vs whodunit

murder mystery and whodunit both are nouns.

Word	Noun	Adjective	Verb	Adverb
murder mystery	Yes	No	No	No
whodunit	Yes	No	No	No

As nouns, whodunit is a hypernym of murder mystery; that is, whodunit is a word with a broader meaning than murder mystery:

murder mystery: a narrative about a murder and how the murderer is discovered
whodunit: a story about a crime (usually murder) presented as a novel or play or movie

Other hypernyms of murder mystery include mystery, mystery story.

murder mystery (noun)	whodunit (noun)
a narrative about a murder and how the murderer is discovered	a story about a crime (usually murder) presented as a novel or play or movie