I actually got surprised myself on another playthrough just yesterday where I was doing pure evil, but she decided to spare the nightsong anyway, which didn’t happen last time I did an evil playthrough. Maybe it’s random in some way, or maybe there’s a dialogue you need to have during some point that I missed this playthrough. I do know if you destroy the grove she becomes clearly upset about it during the party in camp, but I neglected to speak to Shadowheart almost entirely this playthrough so it’s possible I missed some dialogue trigger. I’m not sure myself now