Why would you think Astarion isn't neutral in the first place? He is selfish, mostly cares about his own survival, doesn't use unnecessary violence and if he kills people he does it clean without letting them suffer. That's clearly neutral in my opinion.
Characters like Nere are evil and even among the companions I'd argue that Lae'zel and Wyll are both more evil than Astarion.
Because he's a vampire and that's the D&D lore. You get bitten, your alignment turns.
Clean kills or lack of being a serial killer is not enough for the neutral alignment.
An example: (neutral on chaos/lawful, yet evil)
"A neutral evil character is typically selfish and has no qualms about turning on allies-of-the-moment, and usually makes allies primarily to further their own goals. A neutral evil character has no compunctions about harming others to get what they want, but neither will they go out of their way to cause carnage or mayhem when they see no direct benefit for themselves.
Another valid interpretation of neutral evil holds up evil as an ideal, doing evil for evil's sake and trying to spread its influence. Examples of the first type are an assassin who has little regard for formal laws but does not needlessly kill, a henchman who plots behind their superior's back, or a mercenary who readily switches sides if made a better offer. An example of the second type would be a masked killer who strikes only for the sake of causing fear and distrust in the community. Examples of this alignment include many drow, some cloud giants, and yugoloths."
Wikipedia refers to the player's handbook here, specifically, so I will end my research.
For sake of argument, neither do the Zhentarim kill for the lols and yet they are lawful evil.
"A lawful evil character sees a well-ordered system as being easier to exploit than to necessarily follow. Examples of this alignment include tyrants, devils, corrupt officials, undiscriminating mercenary types who have a strict code of conduct, blue dragons, and hobgoblins."
The question remains what type of evil Astarion is. But, chance of extreme lore violations non withstanding, he must be. There is no evidence he is not /not/ evil. That is enough to conclude his alignment for me personally.
