You assume a lot based on a true vampire lore, I go by what the game tells us. In the end its all open to enterpretations.
I just draw conclusions from his character arc, promo videos with the Larian writers ("If you won't challenge your love interest, they may become worse and as an example of negative consequence may be: they will change you into a vampire") and Astarion's main theme - power and freedom at all cost. It can go right, it can go wrong. The writing itself could be done better - that's true - but the branching of his arc's structure is pretty logical. Vampire lore is only an addition to it.
In the EA Astarion even had a scene in which he said a power, bending people to his will and a little corruption is fun. It was inside him from the very beginning.
