decedent vs someone

decedent and someone both are nouns.

Word	Noun	Adjective	Verb	Adverb
decedent	Yes	No	No	No
someone	Yes	No	No	No

As nouns, someone is a hypernym of decedent; that is, someone is a word with a broader meaning than decedent:

decedent: someone who is no longer alive
someone: a human being; person, singular, assertive existential pronoun; pronoun, person, singular; quantifier: assertive existential

Other hypernyms of decedent include individual, mortal, person, somebody, soul.

decedent (noun)	someone (noun)
someone who is no longer alive	a human being; person, singular, assertive existential pronoun; pronoun, person, singular; quantifier: assertive existential