leviathan vs monster

leviathan and monster both are nouns.

Word	Noun	Adjective	Verb	Adverb
leviathan	Yes	No	No	No
monster	Yes	No	No	No

As nouns, monster is a hypernym of leviathan; that is, monster is a word with a broader meaning than leviathan:

Other hypernyms of leviathan include behemoth, colossus, giant, goliath.

leviathan (noun)	monster (noun)
the largest or most massive thing of its kind	(medicine) a grossly malformed and usually nonviable fetus
monstrous sea creature symbolizing evil in the Old Testament	a cruel wicked and inhuman person
	a person or animal that is markedly unusual or deformed
	an imaginary creature usually having various human and animal parts
	someone that is abnormally large and powerful