I see it as a step towards him reclaiming his power as a free vampire ("after all, you know what I am now, I can fight with all my weapons - teeth included"). He IS a vampire, has been for 200 years, this can't be changed. But whether he becomes a true monster by his actions in the rest of the story, and what he does with that power - that remains to be seen.
In this setting vampires are evil monsters, or at least they were in BG2 (though I don't play pnp, so I don't know how vampirism is handled now). It was similar how dragons were color-coded; for example a red dragon would be evil, as this was their nature. When Astarion chooses to reclaim his power, that is power granted by being a monster, and he chooses to do so by trying to drink blood in secret. He only considers asking for consent after he was caught. Whether Astarion cannot do anything about his vampiric nature, that is unclear. In BG2 there was a quest about reversing vampirism, so it was possible in this setting according to the old rules.
Though I think they will put in a redemption path, because the writers will have to accomodate a player who chooses to play as an evil origin character and yet makes consistently good choices.
