Making Minthara a "good" character would actually takes everything from what makes her interesting and awesome, i actually would love to have her in my party, but i respect her for her personality, not just her looks!
I did not mean to make her good, but the ability to leave her alive with a "good" passage, with the specifics of her relationship with a good character who decided to spare her and help her deal with himself and the tadpole. She will be evil, less evil, neutral with this option - at the discretion of the developers, it is only important that a good hero can try to be good with her, and could win her respect and interest even being good. Or evil, but not enough to stage tiefling genocide
It should be a challenge, it should be very difficult in comparison with the path of evil, and take much longer, but very interesting opportunity for roleplay.
I think it's logical that a character like Minthara or Astarion doesn't want to die when she realizes she's been defeated.