For my canon playthrough where I had no metaknowledge beyond parts of act 1 that I had seen in early access and I played my character to react to things making the most logical choices that fit their personality and what they knew, I sent Astarion away after he tried to bite me. My logical reasoning for this was that I already knew him to be dishonest, so there was too much risk trusting that this time he would stay true to his word.
Trusting him turns out to be a very beneficial decision in the end, but it is objectively a terrible decision at that pont in the game when looking at all the facts you have in front of you.
Like you, I always try to make the choices that best fit my PCs’ characters and what they know. Whether those choices are logical or not will depend on the character
I can see why many good characters would send Astarion away or even kill him after the bite scene, but I’ve now done two chaotic good playthroughs (despite trying to mix it up and play different characters, it is my happy place) and in both Astarion stayed with the party but for different reasons. In my first, my PC wouldn’t kill him once he’d backed off and was willing to extend the benefit of the doubt that he’d not killed before, but sending him off to potentially prey on innocents seemed irresponsible so she thought she’d better keep him where she could keep an eye on him. My second chaotic good PC was an erratic risk taker with a heart of gold, so would always be happy to give folk a second, third and even fourth chance, and was also fascinated by everything and had never met a vampire spawn before so it was an excellent opportunity for new experiences and knowledge (this was my cleric of Oghma/ jack-of-all-trades playthrough).
