Does emotional intelligence promote behavior that strictly benefits the greater good, or can it also advance interpersonal deviance? In the investigation reported here, we tested the possibility that a core facet of emotional intelligence--emotion-regulation knowledge--can promote both prosocial and interpersonally deviant behavior. Drawing from research on how the effective regulation of emotion promotes goal achievement, we predicted that emotion-regulation knowledge would strengthen the effects of other-oriented and self-oriented personality traits on prosocial behavior and interpersonal deviance, respectively. Two studies supported our predictions. Among individuals with higher emotion-regulation knowledge, moral identity exhibited a stronger positive association with prosocial behavior in a social dilemma (Study 1), and Machiavellianism exhibited a stronger positive association with interpersonal deviance in the workplace (Study 2). Thus, emotion-regulation knowledge has a positive side and a dark side.