big bedbug vs assassin bug

big bedbug and assassin bug both are nouns.

Word	Noun	Adjective	Verb	Adverb
big bedbug	Yes	No	No	No
assassin bug	Yes	No	No	No

As nouns, assassin bug is a hypernym of big bedbug; that is, assassin bug is a word with a broader meaning than big bedbug:

big bedbug: large bloodsucking bug
assassin bug: a true bug: long-legged predacious bug living mostly on other insects; a few suck blood of mammals

Other hypernyms of big bedbug include reduviid.

big bedbug (noun)	assassin bug (noun)
large bloodsucking bug	a true bug: long-legged predacious bug living mostly on other insects; a few suck blood of mammals