Psychopathy and narcissism are multidimensional constructs with substantial overlap. Low agreeableness (i.e., antagonism) features prominently in clinical and theoretical descriptions of both disorders. The authors examined whether antagonism components of their assessments accounted for the overlap between narcissism and psychopathy. Next, they tested whether the antagonism components were responsible for the relations that narcissism and psychopathy bore to aggression outcomes. Using multiple regression, the authors found that the low agreeableness component accounted for the majority of overlap between psychopathy and narcissism, nearly all of the variance in narcissism's relations with aggression outcomes, and the majority of variance in psychopathy's relations with aggression outcomes. Disinhibitory traits, which serve to distinguish psychopathy from narcissism, accounted for incremental variance in aggression outcomes for psychopathy. Results are discussed in the context of the overlap between narcissism and psychopathy. The authors argue that low agreeableness is largely responsible for the maladaptive outcomes associated with grandiose narcissism and psychopathy.