causal agent vs killer

causal agent and killer both are nouns.

Word	Noun	Adjective	Verb	Adverb
causal agent	Yes	No	No	No
killer	Yes	No	No	No

As nouns, killer is a hyponym of causal agent; that is, killer is a word with a more specific, narrower meaning than causal agent:

causal agent: any entity that produces an effect or is responsible for events or results
killer: the causal agent resulting in death

causal agent (noun)	killer (noun)
any entity that produces an effect or is responsible for events or results	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