For now, I have concurred and decided to do away with the balancing of Light and Shadow for my Priestess in favor of having her remain a peaceful nice lady.
However, there IS one way I can think that she could, in essence, fall to the dark side and which could theoretically play off the previous idea of not being able to have balance between the two without being consumed by the darker half.
Without going too deep into the subject I can say that the Alliance and Horde have unequivocally become a case of Black and Gray morality over the past two expansions. So let's assume my aforementioned Priestess, after witnessing the destruction of Theramore, the atrocities committed on Pandaria, etc., begins to lose her faith in the Light, and consequently does indeed end up getting consumed by the Shadow & getting branded a heretic and cast out of the church. I was thinking about having her personality shift to being more or less this kind of character: http://tvtropes.org/pmwiki/pmwiki.php/Main/DarkMessiah
Would this trope be a suitable personality for a Shadow Priest in your opinion? Or would she probably still be on the side of Light, just acting more like the Scarlet Crusade?
(One thing worth pointing out is that the example of a Dark Messiah this page gives from Warcraft is none other than Sylvanas)
I assume this is the point right here "with enough good acts and intentions to stay from outright villainy, but he stands on very slippery and muddy ground".
This is more than enough reason to willfully become a shadow priest. The Shadow is all about personal power and yes compassion is one of the lesser taught virtues. But even for any good she does for the Church through the Shadow it will be seen as evil. That's all the Light faith and teachings show. Changing the world for the better is done through helping others (the light), the only way the Shadow would willing aid her is if she was doing this to change the world for herself and believes that completely. No thoughts of I am only doing this to help my old priest friends. The Shadow at it's very core is not about helping anyone but yourself.
They are both faith based religions and as we have seen require a solid devotion (even a twisted one like the Scarlets) to work for their devotees.