Seems nobody has taken this vantage point which I thought would be appropriate now that Vergo is always a buzz that's more interesting than current arc villains. ( no surprise). If we look at the ideology that permeates the World Government it is one where the CDs are gods and the D's are viewed as demons. D's stand for freedom which piracy is the ultimate expression of just that. Demon, devils, these are hypoerbolic terms used for the enemy of the WG, as seen how they branded Nico Robin (the only one with the power to read poneglyphs and thereby capable of unleashing the power that could dethrone the WG) as "Devil Child"; likewise Doflamingo (who blackmails the WG and is seen as the ultimate rogue figure) is branded as the "Heavenly Demon", possessing the knowledge of the secet of Mariegeoise.
Demonic, satanic and devilish allusions are given to the enemies of the WG. You won't find any marine or CP agent with any epithet like that, as per the most notable example, even the ruler of hell itself, Magellan, is not known as "Demon Poison" Magellan or anything like that. (he is actually quite comically known as Diarrhea Man). This is quite a salient fact how Magellan doesnt have any allusions of that nature to his name when he is an actual enforcer of the most hellish system in One Piece.
Reason I believe is it's not politically correct to be named as "devil this" "demon that" if you're an appointe and office holder for the WG, marine, hell, even the ruler of hell itself dont come by such names, and that should tell you something, is because its antithetical to the vocabukaly used to denote the Celestial Dragons. Just imagine if you're wearing a Ted Bundy t shirt in public how people would react even if you're just trying to troll and be funny. That's how WG views the title of "Demon" if you're in their employ. They don't like it.
Vergo is known as Demon Bamboo. He is also quite notably the only marine with an active presence in the NW, heading a marine base that's in the thick of the NW business and going-ons. Reason he has got his epithet is quite simply something he must have been branded with this name by the pirates in the NW who is not touchy-feely about this kind of vocabulary. His strong presence in the NW must have had such a tremendous impact he became known as a "demon" and the name stuck. Vergo is an exception is the point I am getting at. Even WG and marines dont argue with his epithet, that's how goated Vergo is.
NW boss everyone fears him
NW is so butthurt by Vergo (just like this forum) he owns headspace everywhere.
Leader of 300 like Leonidas he is too dangerous