cause of death vs killer

cause of death and killer both are nouns.

Word	Noun	Adjective	Verb	Adverb
cause of death	Yes	No	No	No
killer	Yes	No	No	No

As nouns, cause of death and killer are synonyms defined as:

cause of death (noun)	killer (noun)
the causal agent resulting in death	predatory black-and-white toothed whale with large dorsal fin; common in cold seas
	a difficulty that is hard to deal with
	someone who causes the death of a person or animal
	the causal agent resulting in death