robber vs stealer

robber and stealer both are nouns.

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

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

robber: a thief who steals from someone by threatening violence
stealer: a criminal who takes property belonging to someone else with the intention of keeping it or selling it

Other hypernyms of robber include thief.

robber (noun)	stealer (noun)
a thief who steals from someone by threatening violence	a criminal who takes property belonging to someone else with the intention of keeping it or selling it