executioner vs killer

executioner and killer both are nouns.

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

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

executioner: an official who inflicts capital punishment in pursuit of a warrant
killer: someone who causes the death of a person or animal

Other hypernyms of executioner include slayer.

executioner (noun)	killer (noun)
an official who inflicts capital punishment in pursuit of a warrant	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