That dialogue could be conditional to using the tadpole.
And it's not exactly resisting Shar's power, it's resisting the Nightsong's soul sucking kiss. Kiss that might also be conditional. If Shar is responsible for shielding the tadpole, then she would want you dead if you used it as well.
Makes sense, I agree. An evil goddess protecting the character from another evil entity. Overused concept, but why not.
I hope at some point though, through Gale for instance, Mystra will have a role to play to help Good leaning characters in this fight. Selune too.
Going by the description of what is in Gale's chest,
he's more in Shar's palm than Mystra's right now...
Saying that, before the end of Act 1, you can probably piss off Shar
without using the tadpole or working for the Absolute. As you can remove the shadow curse which Shar wants to keep up and destroy the remains of her power base in the region.
