serial killer vs manslayer

serial killer and manslayer both are nouns.

Word	Noun	Adjective	Verb	Adverb
serial killer	Yes	No	No	No
manslayer	Yes	No	No	No

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

serial killer: someone who murders more than three victims one at a time in a relatively short interval
manslayer: a criminal who commits homicide (who performs the unlawful premeditated killing of another human being)

Other hypernyms of serial killer include liquidator, murderer.

serial killer (noun)	manslayer (noun)
someone who murders more than three victims one at a time in a relatively short interval	a criminal who commits homicide (who performs the unlawful premeditated killing of another human being)