For a different woman, a different relationship, a different situation, gentleness might have been the proper, the only approach--but not for this woman, in these circumstances. The only thing that will cleanse Claire (and reassure her: look at what she says at the end of it. She feels safe again, having felt the power and violence in him) is violence. And--the most important point here--Jamie pays attention to what she wants, rather than proceeding with his own notion of how it should be, even though it's a sensible notion and the one most people would have.