Nobody was sworn to carry your burdens.
Concerning Act 2 Shadowheart
If she destroys the Nightsong, she gets a full set of unique Dark Justiciar armor (boots, gloves, helm, breastplate) and a Spear of Night upgrade. I think they have a bunch of powers that activate while you’re in dim light. There’s also a cutscene where Shar gives her orders to kill Ketheric Thorm.
It’s cool but nothing game-breaking. I’m sure you get equivalent rewards for the opposite decision.