To be honest, I feel like Minthara features, as a character, a huge potential both to be reedemed or to be drawn into the darkness.
On one side, she is clearly willing to harm innocents and acts really nasty towards her followers. She is really, really vengeful as well.
But, if we take look at her other character traits, she can show mercy, she does clearly care about the PC, shows him/her plenty of compassion and trust, while literally risking her life and position, 2 of things that she values the most, for us. But there has to be something specific, for this to happen.
As for the reason, why the romance is going so fast, it's because:
To sum it up; She is multi-layered, complex character, who features both bad and good traits. The reason for why though, the "evil" side does dominate, is written above. Personally, I think that she should feature possiblity of either becoming someone much worse/better; depending on the choice of player, simply because there is a good logical reason for this to happen.
EDIT: Just forgot about it; The whole character of Minthara is a good example why DnD alignment system is flawed. It's simplyfying the possible personalities way too much. For example someone can be mostly selfish, but will never harm others, lie, cheat etc. Basically all of things that make a character good; with exception that they are... selfish.