Is a goddess's evil all that there is to her? We would say "no" about almost any human, and Toril's deities are actually ascended mortals.
So, Shadowheart might not be brainwashed, but right. From her perspective, following Shar may appear right in a way we would recognize as valid could we adopt her perspective. With no need for any evil on her part.
Many are ascended mortals -- AO has reasons for wanting mortals in certain positions and he changed the tablets of fate to ensure that Deities answer to their worshipers BUT Shar and Selune aren't ascended mortals. They were born as gods. They were the first two and haven't changed that much since their initial battle:
In order of appearance:
1) Shar 2) Selune 3) Chauntea
Chauntea has changed over years from a goddess of the wild places to a goddess of gardens and cultivation but is still born as a god and thinks like like a god. Of all the gods Shar has probably changed the least over the millenniums.
