I want to be a vampire, like you. (selfish, transactional)
I want you, I want your body. (selfish, transactional and completely insensitive!)
You can tell me that you've learned something from all this. (Preaching and hostile)
I am not your pet. (hostile)
.
This is a great outline of the dialogue options during post-Ascension discussion where it becomes problematic. It's seems like the writer(s)/director(s) wanted to make sure you feel bad for helping ascend Astarion. Like they are directly calling you selfish or stupid.
All of your points are excellent.
If we are meant to feel "bad", I did not. And the majority of ascending fans did not. They found it to be his favorable ending. Even if it's his "evil" one.
As a writer, you can't force your audience to suddenly align with the writers morals for a major arch in the game. You can't force the audience to share your morals at all. Or your values, and your feelings over a certain occurrence.
If this is indeed a roleplay game, acknowledging that a good portion of the players who are likely playing evil aligned aren't going to suddenly out of thin air have an attack of conscience and think what they did was the wrong thing to do. Or regret it. Or do anything but want to celebrate and revel in their wicked decisions.
Why would you not acknowledge the more realistic reactions of the audience through proper dialogue? What is the purpose of instead telling the audience there is only one acceptable way to feel about their chosen path?
Tldr; knowing your audience > telling them how to feel. Larian failed here. A point to improve on in the future.