I think freedom in this context is usually a euphemism for any hierarchical system that subsumes the individual will, Lawful alignments believe in a reciprocity of actions regulated by a third-party, in a society these are Laws, personally it's some kind of personal ethic that regulates your interactions with others and which you expect to see reciprocated. Chaotic alignments see this as a pretense towards tyranny, and believe there can be no law, third-party, or ethic that can be expected to replicate such reciprocity. So Astarion can have a personal code that totally self-interested and it doesn't make him Lawful, because he doesn't see himself as being subject to a external hierarchy. The difference for a Neutral character is that they don't believe in hierarchies per se but in unstructured principals that dictate actions without subsuming an individual's will, but also don't view the gratification of your own will as an ends unto themselves.
Did I mention how murky this is?
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.
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.