serial killer vs serial murderer

serial killer and serial murderer both are nouns.

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

As nouns, serial killer and serial murderer are synonyms defined as:

serial killer and serial murderer: someone who murders more than three victims one at a time in a relatively short interval

serial killer (noun)	serial murderer (noun)
someone who murders more than three victims one at a time in a relatively short interval	someone who murders more than three victims one at a time in a relatively short interval