Astarion's first instinct is to kill you (well, he's a confused vampire who has just been abducted and subject to mind-flayer hospitality, cut him some slack). Not the best way to introduce a companion you are supposed to like to admire, but that's not really the reactions that Larian expect you to have at this point. I should say that, in D:OS 2, I often killed Sebille thanks to her similar less-than-auspicious introduction.
I agree that the possibility should exist for a better dialogue path at that point though. My take is that the choice allow you to be either be naive to the point of stupid, mockingly hostile, or passive aggressive. A PnP character would deserve to be attacked with those responses.