There was something going on here that she was missing, beginning to wonder if this was just about painting. Maybe he was really out to ruin her life more than she realized. "Because you won't let me leave," she illustrated the point by tugging at her arm again, returning his displeased look with a quietly challenging one.
"No, keep it. Or don't. I don't care," Mabelle shrugged indifferently, although secretly preferring that he did keep the painting for whatever reason that he still had it. Maybe part of her liked the idea that there was something of hers that he couldn't bring himself to burn, that he still couldn't get rid of. That there was something he held onto that would always serve as a reminder of her, no matter how much he disliked her, and that was enough of a victory to Mabelle.
"I'd ask you to cut off more of my fingers, but then it really wouldn't be the same, and I wouldn't be much help to you anyway," she offered unhelpfully, no intention to actually make this any easier for him.