Nobody was sworn to carry your burdens.
There are multiple ways for Shadowheart to go during that scene, depending on how you have been interacting with her up to that point. I’m on a ‘good’ run, and I have maxed out her affinity. I didn’t have to roll any dice. I only had to do something that is pretty obvious, if the choice is there. I guess if you haven’t been going that route, convincing her not to kill said personage is quite hard.