I fail to see how her being something other than human caused her to be a mindless drone.
Like the other people have said. Its not her becoming a vampire literally thats the problem. She could have become a witch, some other mystical creature, whatever else. Its that she went from a decent LI with somewhat of an interesting character and interactions and after she was bedridden and turned her character made a 180 and suddenly can't do a fucking thing without the MC, turned into a princess in a castle that needs rescuing. Its just shit writing.
She had potential to be someone the MC could have relied on with her staying human, to keep the "human" part of the MC alive while struggling to grow his powers, and instead is just another notch for the MC to fuck now. Its what the story could have been, rather than just a gotta fuck em all game.
The first few updates the game seemed to want to tell a decent story rather than what we have now.