It makes sense that Shadowheart would stray from the path of evil on her own, given that she is a good person at heart.
However:
as people pointed out, it makes no sense that if you gently suggest she not commit murder, she threatens to murder you, even if you are in love or have 100 approval. And then you have to pass extremely high persuasion checks or she will try to kill you or kill the nightsong. But if you say to hurry up and kill her, she doesn't snap at you and immediately does as you say. It makes no sense.

How to fix it:
1.) Add a bunch of dialogue once the trials start. You and her should go through Shar's ideology and history in more detail and get into heated moral debates about it while you are down there. That would serve to flesh out her doubts, the lore of the mission, and your influence on her.

2) Tie the options in the final confrontation with Nightsong to your relationship with her, your good/evil actions up to this point, and how much or how little you have challenged(reasonably) her Shar faith. If you have done good deeds, like saving the Tieflings, and you leave the decision to her, she should spare Nightsong. If you did evil deeds like kill the Tieflings or gave up Isobel, she should kill the Nightsong, if left alone. It would be up to Larian to decide if they are going to track a series of decisions as morally good/bad or just hinge the decision on whether you kill the Tieflings at the Grove or not.

If you've done good deeds and have high approval with her, she should easily accept your suggestion to spare Nightsong, without anger or protest. If you've done evil and have high approval with her, she should easily accept your suggestion to kill Nightsong.
If you've done good and have high approval with her but you want to convince her to kill Nightsong, it should be a moderate check(~15). If you've been good but have low approval with her, the persuasion check should be very high (30+).
Vice Versa for evil.

I am sure they will be looking to update dialogue in the future for underdeveloped areas. I just hope that this area gets a complete makeover.