sneak thief vs stealer

sneak thief and stealer both are nouns.

Word	Noun	Adjective	Verb	Adverb
sneak thief	Yes	No	No	No
stealer	Yes	No	No	No

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

sneak thief: a thief who steals without using violence
stealer: a criminal who takes property belonging to someone else with the intention of keeping it or selling it

Other hypernyms of sneak thief include thief.

sneak thief (noun)	stealer (noun)
a thief who steals without using violence	a criminal who takes property belonging to someone else with the intention of keeping it or selling it