I love Astarion BECAUSE he is an evil bastard. I have no wish to change him, he is too much fun the way he is. His sense of humor is wholly inappropriate, he provides not so subtle reminders that we need to see to our own interests first and he has no moral qualms about any path we choose as long as it is successful.
Frankly he is the least "whiney" character we have, and his Arc is about revenge, which is always fun. On top of that he is a decently made rogue with enough versatility to make him incredibly strong. My one complaint is his low strength score. I would personally never make any character with strength that low - should be 10-12 minimum.
But I digress. Going into any relationship with the idea that "I can change him/her" is a recipe for disaster. Any therapist will tell you that you can't change anyone unless they choose to seek to change themselves.
Oh and Silver is 100% right,
Vampire Spawn are Evil. They are infused with energy from the negative material plane, which makes the evil Intrinsic, not cultural. While he remains a Vampire Spawn his alignment
cannot change.
So maybe that is what you should be looking at in regards to his redemption Arc. However, only a Wish spell OR a True Resurrection Spell could really do that.
Good luck!
