I don't think he becomes stupid all of the sudden - although maybe the writer could prepare this situation a bit better. At first Astarion is in a uncertain place - it's not clear how he will change (and let's remember he is not a good person from the start). He wants power and when he gets it (again, commiting mass murder as a result of devilish pact with an archdevil)- he corrupts it. This is the consequence of evil choice.
Well, becoming evil wouldn't make you stupid all of a sudden or become irrational and loose all logical thought. It can corrupt your intentions and feelings I suppose, but not make you an idiot. The 6 spawn are not his real family as I understand it, Casador insisted that they called themselves a family, Astarion states it himself in a conversation.
