plagiarizer vs stealer

plagiarizer and stealer both are nouns.

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

As nouns, stealer is a hypernym of plagiarizer; that is, stealer is a word with a broader meaning than plagiarizer:

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

Other hypernyms of plagiarizer include thief.

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