In response, their collective rage, although perfectly justified, was unharnessed. They killed men whose indifference to Shechem's violation of Dinah betokened their silent support of his dastardly act. Had they confined their rage to the culprit, perhaps Yaakov Avinu might not have been as critical of them. Rage cannot catalyze a blessing. The natural consequence of their impulsivity was a curse.
Unbridled anger is a character trait that renders a person unfit for leadership.
Horav Hirsch notes the significance of Yaakov Avinu's words to the emerging nation. At the laying of the cornerstone of the Jewish nation it was of profound importance to emphasize that every violation of the laws of morality and justice, even those performed in the best interests of the community, produces a curse. Cunning, guile and brute force, which in private life never go unpunished, result in accolades and civic honors in the public sector. The laws of morality exist only for private life. In politics and diplomacy, the only code of ethics that is recognized is the one that supports the party or the state.