Integrating self-uncertainty management and thwarted needs perspectives on leader mistreatment and workplace deviance, we examine when and why leader mistreatment is associated with workplace deviance. We propose that competence uncertainty strengthens the relationship between leader mistreatment and workplace deviance and that hostility mediates this interactive effect. Four field studies and one experiment support the hypotheses. The first two studies provide evidence for the predicted interaction between leader mistreatment and competence uncertainty, and the next three studies demonstrate that hostility mediates this interactive effect. We discuss an extended social exchange explanation of workplace deviance and highlight the psychological interplay between motives, cognition, and affect in reciprocating leader mistreatment.