none but the devil
Mary Shelley characteristically complicates the moral lines here. Even as Justine is called a "monster," she resorts to her own habitual modes of religious instruction to categorize the behavior of the murderer. That she happens to use the same terminology as Victor does is a nice irony. But if we then seriously accept his own claim of ultimate responsibility for this debacle, Justine's invocation of the devil is tantamount to an ironic accusation against Victor, implicitly inverting his earlier (I:3:8) moral exoneration of himself as a godlike creator.