If you decide to defend the Grove - where I feel like some protection for her is necessary due to all of the explosives alone - there could be a persuasion roll to tell her that with such a weak army she will never do her Goddess's bidding so she cancels the attack early. This would be in line with her argument in the throne room.
A narration line, that reminds your character that basically all the true souls are brainwashed and how easily you could have been one of them might also help. After all why do you spare her, when you kill the other leaders? For it to make sense in the story, there needs to be a reason apart from her being a potential follower. Do you spare her out of empathy? Because she might be useful later on? Or are you maybe a little bit impressed by her and simply don't want to see her end that way?
