That's what happens to someone who becomes a true vampire in dnd. The game tells you that in Act 1. There is a reason why ascending is the "bad" ending for Astarion's personal quest. Doesn't matter what kind of person he was before.
And I would also love him to be as OP as expected, but that would break the gameplay.
Oh no... you mean "evil" not "bad". He gets everything he aspired to as a corrupt magistrate. You reminded me of something else. Please bring back the damn corrupt magistrate. Bring back his past at least with documents or newspapers. Don't build his whole story around the fact that he's just a victim of Cazador. Early Access players know who he was.