The effects of guilt, shame, and externalization of blame on aggressive behavior were investigated among a total of 307 Finnish fifth and sixth graders (Mage = 11.9 years). Self-reported proneness to feel guilt and shame was expected to reduce levels of peer-reported aggressive behavior, whereas self-reported externalization of blame was hypothesized to function as a moral disengagement mechanism with links to greater aggressive behavior. However, these associations were expected to be moderated by children’s emotion-regulation capabilities and tendencies to experience negative emotionality (as reported by teachers). Results indicated that guilt and shame were associated with lower levels of aggression for children with poor emotion regulation (or high negative emotionality). For children with effective emotion regulation (or low negative emotionality), shame and externalization of blame were associated with higher levels of aggression. The results suggest that a dark side may be apparent in effective emotion regulation (and low negative emotionality) in that it enables children to disengage from the normally inhibiting functions of guilt and shame and to act aggressively in response to shame and externalization of blame.