Research on customer mistreatment has focused on victims’ perspectives with scant research on third-party observers’ reactions, especially their reactions to customer mistreatment of supervisors. Drawing on the deontic justice theory, this research investigates how and when customer mistreatment of supervisors impacts frontline employees. Using a mixed-method of three studies (experiment, time-lagged survey, field survey) from two countries (the U.S. and China), this research demonstrates the spillover effects of customer mistreatment of supervisors on employee’s service sabotage via moral anger. Study 1 showed that hospitality employees’ moral anger mediated the relationship between customer mistreatment of their supervisors on employees’ service sabotage. Furthermore, Study 2 and Study 3 provided empirical evidence supporting the moderating role of moral ownership. Employees are more likely to respond to customer mistreatment of their supervisors with moral anger and service sabotage when they have high, rather than low, moral ownership. Theoretical and practical implications are discussed.