Lily’s decision to grab a knife reveals that her anger at her father and the need to step in for her mother now take precedence over rationality or safety; it’s only at this point that Atlas takes the situation out of Lily’s hands. In this scene, Lily sees her father in absolute, black-and-white terms. He tried to rape her mother, and she wants to hold him accountable—she knows her mother won’t let that happen, so in many ways, Lily unfairly blames her mother for her father’s violence. Because Lily was livid at her mother for letting any of this happen, she interpreted her mother’s failure to acknowledge Atlas when she came upstairs as yet another instance of denial. Regardless of whether or not this is true, Lily’s conclusion at the time shows exactly how weak she believed her mother to be.