Whether she kills or spares Aylin serves the interests of the party, since Ketheric loses his protection either way. And you know this before going in. So for a (lawful) good character stopping SH at the nightsong before she makes a decision probably makes the most sense. But I don't see a good reason for other tav's to intervene here.
Sparing Nightsong could be the moral thing to do, but it is probably not in the interest of the party.
Ketheric can't be defeated so long as he is immortal, and his immortality depends upon Nightsong. We know that Nightsong has been defeated and captured once already, and allowing her to live is a significant risk: If captured again, the chance to defeat Ketheric might be lost.
And that scenario almost comes to pass if Nightsong is spared - she is recaptured during the rooftop battle, and Ketheric regains his immortality until she is freed again.
Heck, given her solo flight to Moonrise Tower, and the party's long fight to reach Ketheric, it's rather lucky she wasn't defeated and secreted away somewhere (perhaps aboard a departing nautiloid) before the rooftop battle. Her temperament is largely unknown, and the player doesn't have the option to request that she remain with the party for safeguarding if freed (a minimum precaution).
A game over at this point for parties who spared Nightsong wouldn't be outlandish given their gambling of both their own survival and the fate of the world for the sake of doing "the right thing". And this is overlooking that defying Shar in her own realm might be a fatal mistake to start.
Luckily for Lawful Good Paladins everywhere, Larian allows "good" choices to be made without suffering much if any logical consequences while "evil" choices are punished harshly. The cost of sparing Nightsong is more-or-less "Misty Step + Help" - and then she joins in the battle to boot.
All of this is to say that the scene was handled poorly in general, not just in regard to Shadowheart's characterization. There's no real back-and-forth discussion of potential consequences or morality among the party despite the gravity of the situation, nor does Nightsong argue much of a case for herself.
