Even if this direction was inevitable once they'd met, it's never going to not be a mistake. Kitty has had too many bad relationships and might be too willing to accept disaster; Ben hasn't had any and, in addition to his general instability, he has very little understanding of what constitutes a healthy relationship with another person. He's aware of how terrible it is to foster an attachment between them, but he likes being around Kitty. He isn't willing to sacrifice their connection.

"I wouldn't try to kill you now." As if that makes it okay. Maybe, in both of their minds, it does. "Don't be afraid."

She has no reason not to be afraid that this will go poorly.