kidnapper vs criminal

kidnapper and criminal both are nouns.

Word	Noun	Adjective	Verb	Adverb
kidnapper	Yes	No	No	No
criminal	Yes	Yes	No	No

As nouns, criminal is a hypernym of kidnapper; that is, criminal is a word with a broader meaning than kidnapper:

kidnapper: someone who unlawfully seizes and detains a victim (usually for ransom)
criminal: someone who has committed a crime or has been legally convicted of a crime

Other hypernyms of kidnapper include captor, capturer, crook, felon, malefactor, outlaw.

kidnapper (noun)	criminal (noun)
someone who unlawfully seizes and detains a victim (usually for ransom)	someone who has committed a crime or has been legally convicted of a crime

kidnapper (adjective)	criminal (adjective)
	guilty of crime or serious offense
	involving or being or having the nature of a crime
	bringing or deserving severe rebuke or censure