To me, murdering people in games, for the sake of 'goodness' is still murder. Whenever I play D&D games I like to think things through (like 'what would my character do in this situation?') and I do my best to try to talk my way out of difficulties rather than attack on sight (that's what zealots do), or in case of characters like Astarion/Shadowheart give them the benefit of doubt.
You, as the player, know that Astarion is a vamp spawn or that Shadow is a priestess of Shar, but your character doesn't.
Considering the events of the day with an abduction by mindflayers and a little trip to avernus? My character is really not in the mood to give anyone the benefit of the doubt. My mage has studied the arcane and the various gods and their symbols. Why would it be metagaming for him to recognize the sharran symbolism on Shadowhearts armor?
You can choose to play the suspicious type of person, all the more power to you. You are free to play however you wish. All I said was my reasoning behind my decisions. But to be honest, there should be some arcane/religion/other skill checks to figure out Shadowheart, Astarion and Wyll (mkay in Wyll's case if you are playing a warlock yourself you can confront him about it) long before they open their mouths about their secrets.
I think this is more a problem of marketing and the origin character concept than a problem with the game itself. In the game itself all these details about our companions played out like some mystery that gets revealed as the game progresses (true for all the companions, but especially true in the case of shadowheart, Wyll and Astarion). But the marketing spoiled them completely and when the game treats these revelations as big surprises it's just jarring
But to be honest, there should be some arcane/religion/other skill checks to figure out Shadowheart, Astarion and Wyll (mkay in Wyll's case if you are playing a warlock yourself you can confront him about it) long before they open their mouths about their secrets.