Scant research has investigated the potential influence of personal relative deprivation on moral disengagement, and little is known regarding the mechanisms underlying this relationship. Inspired by relative deprivation theory, this study proposed a moderated mediation model to explore the relationship between personal relative deprivation and moral disengagement by examining the mediating role of malicious envy and the moderating role of Honesty–Humility. Five hundred and eighty-nine Chinese adults completed anonymous measures of personal relative deprivation, malicious envy, Honesty–Humility, and moral disengagement. Results revealed that personal relative deprivation was positively correlated with moral disengagement and that malicious envy fully mediated this relationship. Moderated mediation analysis indicated that Honesty–Humility moderated the relationship between personal relative deprivation and malicious envy and that between malicious envy and moral disengagement. Specifically, both relationships became weaker for adults with higher levels of Honesty–Humility. These findings not only highlight the importance of identifying the underlying mechanisms between personal relative deprivation and moral disengagement, but also provide valuable implications for the effective prevention and mitigation of moral disengagement.