Again, I didn't in any way, shape or form implied, he is lying here, I think, he is answering to the best if his knowledge. And I think, it is established, that he was a magistrate when he was alive,so yes, he has a knowledge of mortal laws at least. If I would be mad at Astarion about lying about his past in the beginning, that would be pretty petty, since obviously no one will go around telling everyone that they are a vampire spawn. Plus I would have to be mad at Gale and Shadowheart and even our local hero Wyll too, since their secrets are potentially more life threatening than a single small spawn. And apart from his past, Astarion did to my knowledge never lie. After his secret was out, he was pretty open with everything, even, when he knew, he could potentially make enemies with that.
And yes, the killing of 7000 people is a deal breaker for me personally, especially since Sebastian and the Gur kids are pretty decent. The other deal breaker is killing the Gur, since I like them. But I take the devil deal into consideration, since I like to think things through. It is not on my priority, since it won't be me getting dragged to hell, but I can't help, but thinking about that, after having devil deals nearly through the whole game and most of them are at least somewhat harmful.
I just read a post on Reddit from an AA fan, who said, she was verbally attacked and even received death threats. If that is the climate, you guys have to discuss, then I do understand, why you are a bit jumpy and I'm really sorry. This is a game with fictional characters and everyone should play as they like and if some stupid idiots don't understand that and threaten people, they just destroy the fun for everyone. That is not ok.
"We are all stories in the end. Just make it a good one."