Are you saying that Will Power equates Combat Strength? The Knave could've easily killed her, but she chose not to after realizing that Furina didn't have the gnosis. Furina herself admits during her SQ that she has no Combat Prowess prior to obtaining her Vision. In Psychology, Will Power can be defined as "resisting short-term temptations in order to meet long-term goals" and in Furina's case, that "short-term temptation" would be to confide to someone and share her burdens, yet she chose to shoulder everything on her own for the "long-term goal" of defying the prophecy.

Furina after her SQ can, to an extent, take care of herself. But it's also true that the Knave's attempt at her life has scarred her. If someone tried to kill you, easily could have, but decided in the end not to do it because "there's no point in it" doesn't leave some kind of deep negative impact on you then idk if ur ballsy or just naively believe that it won't affect you unless you actually go through the ordeal.