Honestly I had trouble figuring her out too.
On one hand she seems to value trust and loyalty. Me not selling out Astarion got her to flirt with me.
On the other hand trying to help the tieflings and doing the morally right thing is something she disapproves us.
So I guess as long as I am loyal to her and don't betray the trust of my friends she doesn't care what happens to others?
You forgot Astarion is Evil. She is expecting you to do evil things that are convenient for the evil gang. That's why she approved of ransacking the vault too. She didn't want you to save Lae Zel because she was expecting to be the only one manipulating you and doing her bidding. It's obvious. There is nothing to figure out.
