stealer vs plagiarizer

stealer and plagiarizer both are nouns.

Word	Noun	Adjective	Verb	Adverb
stealer	Yes	No	No	No
plagiarizer	Yes	No	No	No

As nouns, plagiarizer is a hyponym of stealer; that is, plagiarizer is a word with a more specific, narrower meaning than stealer:

stealer: a criminal who takes property belonging to someone else with the intention of keeping it or selling it
plagiarizer: someone who uses another person's words or ideas as if they were his own

stealer (noun)	plagiarizer (noun)
a criminal who takes property belonging to someone else with the intention of keeping it or selling it	someone who uses another person's words or ideas as if they were his own