Motivated by theoretical accounts positing that participation in intergroup conflict is driven by a desire to promote the in-group, past studies have explored the link between prosocial personality dimensions and out-group harm. However, while dimensions such as Honesty-Humility predict in-group cooperation, they do not explain out-group harm. Across two incentivized experimental studies (one preregistered; overall N = 1,584), we show that out-group harm is uniquely associated with higher levels of the Dark Factor of Personality (D), a personality dimension capturing the core of all aversive personality characteristics. Conversely, high levels of D, alongside low levels of Honesty-Humility, are associated with less in-group cooperation. Our results show that in-group cooperation and out-group harm are associated with distinct personality dimensions.