robber fly vs bee killer

robber fly and bee killer both are nouns.

Word	Noun	Adjective	Verb	Adverb
robber fly	Yes	No	No	No
bee killer	Yes	No	No	No

As nouns, robber fly and bee killer are synonyms defined as:

robber fly and bee killer: swift predatory fly having a strong body like a bee with the proboscis hardened for sucking juices of other insects captured on the wing

robber fly (noun)	bee killer (noun)
swift predatory fly having a strong body like a bee with the proboscis hardened for sucking juices of other insects captured on the wing	swift predatory fly having a strong body like a bee with the proboscis hardened for sucking juices of other insects captured on the wing