>>536038497
I didn't feel like repeating much of what Mintharafag already said in the post linked above. But sure, I'll elaborate.

The gist of it is that choosing to kill them out of mercy is playing right into Shar's hands. The ethos of Shar revolves around forgoing parts of yourself - your memories, your connection with others, your emotions, even your capacity for love. Shar tells you that numbing the source of your suffering in that way is worth losing yourself - but suffering is inherent to existence. Running away from this or that pain may temporarily work (people do it all the time in real life by abusing drugs), but suffering never goes away. It's something you accept as part of living.

>Try to exclude the possibility of suffering which the order of nature and the existence of free-wills involve, and you find that you have excluded life itself.
- C. S. Lewis, or maybe Alfira.

The second angle is that saving her parents is what Shadowheart wants, as she clearly and repeatedly tells you. Understandably, the circumstances of the moment make her doubt and waver, and I'm not saying it's an easy, clear-cut choice for her to make - or for you to influence as a friend. Still, stack up the pros and cons from a cold and logical perspective, if you must, and it becomes clear that saving them is preferable for Shadowheart herself. The alternative might not appear "wrong" per se, but I wager this is because the writers did not want it to be more depressing. Saving her parents is the "correct" choice because it is life-affirming.