I really disagree with OP here, while I don't particularly like Shadowheart, I don't think she is bad, and her character and actions are very understandable. Also it is very human to do something and then only realize the exact scope of it when you are finally left with the consequences, it is called regret.
To the better discussion...
Wizards kinda do be like that though, going for amazing cosmic power thinking they can handle it and then getting way over their head. My 4e Wizard character unintentionally got involved with demons and while he has (kinda) overcame that to become stronger, said involvement endangered the party and continues to endanger the party. Gale comparatively is a walking time bomb because he got involved with things he shouldn't. Wizards delve into the power of magic and get stronger if they overcome falling to it, or they cause great unintentional disaster.
Astarion I don't think he is completely evil, like he is clearly on the evil side, that is to be completely certain, but he has his moments and I think involvement with our characters will give him a a path to semi redemption
