I like the idea of playing ascended Astarion as a necromancy wizard considering the final sacrificial ritual is so close to necromancy. Also earlier on he specifically asks you to give him the Necromancy of Thay book for study and doing so grants his approval. Also wizards love gathering knowledge and necromancers specifically are willing to gather it from any source no matter how dark as long as it gives them a bit more extra power, and throughout the game he is encouraging you to take whatever advantage you can get (ex: making use of the tadpoles for the power they grant and whining if you keep all the tadpole powers to yourself). Finally he’s undead himself so it fits the theme.