Saying she is evil because she seems to be sworn to an evil god is a real slippery slope.
Nope. Considering a Shar-worshipper to be evil is an eminently reasonable, proper and correct thing to do in the context of the Forgotten Realms setting. NOT considering a Sharran to be evil, and excusing and rationalizing their being a Sharran is what's a slippery slope.
Now, based on what I've seen from both my play-throughs and various videos, it appears that Shadowheart may not actually be evil. There are situations where you literally read her mind and her thoughts are more appropriate for a good character than an evil one. At the same time, you're absolutely right in that she's a cleric of Shar and that means evil.
To me that means the only logical course of action is to start off by considering her as evil, regardless of what I find out later. At the beginning, the only valid opinion a character can have is that she's evil. As other evidence comes in, it's perfectly reasonable to question whether she's actually evil, brainwashed, or something else based on the evidence. But if you accept the later evidence that she might not truly be evil, you have to accept the initial evidence that she should at the very least be treated as evil when you first meet her.
