I think neutral evil characters believe in a hierarchy but they are (supposed to be) on top. After all they put themselves first. If you put something first that's already a hierarchy.
What you describe here is lawful evil. Lawful evil believes in the existence of a strict heirarchy and rules but also strives to bend those rules for their own ends so they can be at the top of that hierarchy. Devils are the quintessential beings of lawful evilness in Forgotten Realms if you want to see what that alignment is all about. Brokering one-sided deals and upholding people to their agreements knowing full well you conned them into making said agreement.
A Neutral Evil character cares about themselves first and foremost but don't have a particular like of rules and order nor seek to break down rules and order when they find it.
Lawful Evil is the career politician while Neutral Evil is the shady hit man the politician hires to take out rivals.
I firmly believe Astarion to be Neutral Evil, the reason why I mention lawful a lot because he has some lawful and some chaotic tendencies, but he is not really on either extreme.
Astarion is not lawful at all. He's pretty definitively a chaotic evil character. He loves causing pain, loves it when you kill indiscriminately or start unnecessary fights, and likes to provoke people into violence. All without an actual end-goal beyond his own personal enjoyment. He also complains when you diffuse situations and avoid conflict even if those conflicts would have only hindered your quest to deal with the tadpole.
He has basic manners and the good sense to not piss off his allies, but that is a long way from being lawful. He's just not full meme-tier chaotic.
