regicide vs killer

regicide and killer both are nouns.

Word	Noun	Adjective	Verb	Adverb
regicide	Yes	No	No	No
killer	Yes	No	No	No

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

Other hypernyms of regicide include slayer.

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