Prior work on supervisor bottom-line mentality (SBLM) has suggested it represents a static, unbending focus, with supervisors so focused on the bottom line that they discount ethical considerations. We propose that SBLM varies, within-person, given various factors in a supervisor’s work life that pull and push their attention to and away from the bottom line across their workweeks. We theorize that the varying nature of SBLM elicits anxiety in employees that is exhausting because, on the days supervisors give greater emphasis to the bottom line, employees must abandon the comfort of their routines to produce bottom-line results. Ultimately, this experience motivates employee unethical behavior (i.e., coworker undermining). We also predict that, by providing employees support and guidance, supervisors’ steadfast commitment to ethics (i.e., between-person ethical leadership perceptions) influences the degree to which exhausted employees undermine their coworkers. Results from three experience-sampling methodology studies using diverse samples of working adults support our predictions. In Study 3, we also test assumptions in our theorizing with research questions about potential antecedents of SBLM variability and the moderating effects of ethical leadership. These results revealed that supervisor daily ratings of their leader’s BLM and the supervisor’s own job demands prompted SBLM variation. In addition, supervisor daily ratings of their own BLM were related to employee-rated daily SBLM. Second, while ethical leadership varies within-person, within-person perceptions did not moderate effects; only between-person perceptions (or employee perceptions of their supervisor’s general commitment to ethics) did. Implications for theory and research are provided.