MY THEORY is that she's ACTUALLY a cleric of Selune, but Sharrites wiped her memory and turned her onto their religion. She's actually a good person, under there, and depending on the player will embrace Shar or rediscover who she was
Larian need to give her a pair of glasses and a hair bun. Then, when she changes personality to be a better person, she can whip the glasses off, undo her hair, and the PC can say, "Why, Miss Shadowheart, you're really a beautiful person."
Of course, it might be a double transformation - pretending to be a cleric of Selune pretending a cleric of Shar, but actually an agent of Lolth. The hostility with Laz'el is all just part of the deception plan.