killer vs cause of death

killer and cause of death both are nouns.

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

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

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