I have the same problem with this scene. None of my Tavs have a good reason to trust Shadowheart at this point; they're extremely reluctant to even help her with the Sharran mission she's on, but they figure they can stop her if they need to. Then the only way to stop her is... not to stop her?
Headcanon to the rescue, I guess.
My good-aligned Tav would do everything he could to stop her from doing the wrong, evil thing. If this actually causes her to be more inclined to do the evil thing, and she goes ahead and does it, then my good-aligned Tav will execute her for her crime right then and there.
