dead body vs roadkill

dead body and roadkill both are nouns.

Word	Noun	Adjective	Verb	Adverb
dead body	Yes	No	No	No
roadkill	Yes	No	No	No

As nouns, roadkill is a hyponym of dead body; that is, roadkill is a word with a more specific, narrower meaning than dead body:

dead body: a natural object consisting of a dead animal or person
roadkill: the dead body of an animal that has been killed on a road by a vehicle

Other hyponyms of dead body include kill, carcase, carcass, carrion, cadaver, clay, corpse, remains, stiff, mummy.

dead body (noun)	roadkill (noun)
a natural object consisting of a dead animal or person	the dead body of an animal that has been killed on a road by a vehicle