It would be very convenient if Promise's ethical opinions were such that rescuing her from Thorn and getting her her tree entitled the prince to one free assassination, that having exhibited an objection to strangers drowning other strangers and her being without something it took him a few seconds to fetch he could be safely issued a license to kill.
Her ethics are not thus, so she has to think.
What would have to be true for this to be an elaborate deception -
- for one thing he'd have to believe he wasn't actually obviously in the right, that his actual case was weak and he needed to make up for it with not even particularly lurid allusions to torture. He's not optimizing for sounding convincing or urgent, though.
That flash of horror when he rescinded her orders would have to be fake, composed very quickly, composed without knowing anything about her or even fairies in general besides that she'd been held by someone who hurt her -
It's possible he's a mindreader and lied about that to tailor his pitch but she was not exactly thinking very clearly when he originally rescued her so she couldn't have been originally filtered for the purpose even like that.
Okay, he's not lying; could he be wrong -? Unlikely; if his story is true-as-he-understands-it he was paying attention and he seems very - meticulous.
She writes herself in circles for a bit, decides to sleep on it, does that, wakes up, eats haws that taste like safety all alone in her very own tree -
- comes out and says, "I'll help you."