The Curse of Hatred (憎しみの呪い, Nikushimi no Noroi) is an ideal common to members of the Uchiha clan. According to Tobirama Senju, the Uchiha are people greatly devoted to love and friendship, yet they tend to conceal their more affectionate traits. When the object of their affection is lost, an Uchiha's love can instantaneously turn into hatred, leaving them more inclined to do anything and everything in their power to achieve their goals and show their own superiority, regardless of the consequences and repercussions of their actions. The despair of these experiences cause a manifestation of unique chakra within an Uchiha's brain, impacting the optic nerves and creating the Sharingan. The Sharingan's advanced form also requires extremely painful experiences, which cause the user to delve into darkness that utterly consumes them.
The Curse of Hatred was first displayed by the Uchiha's ancestor, Indra Ōtsutsuki. When Indra's father, the Sage of the Six Paths, was on his deathbed, he was left to choose which of his two sons would carry on his legacy. Indra believed peace could be achieved through power, while his younger brother, Asura Ōtsutsuki, believed peace could be achieved through love. The Sage ultimately selected Asura. Indra was overwhelmed with anger for being passed over and, once tempted by Black Zetsu, waged war against Asura to claim what he believed was his birthright. Their battle would pass through the generations, continued by their descendants: Indra's Uchiha clan and Asura's Senju clan.
The Uchiha's Curse of Hatred can be considered a counterpart to the Senju's Will of Fire. Whereas the Will of Fire encourages a selfless approach to life, sacrificing oneself for the betterment of the many, specifically one's village, the Curse of Hatred typically sees Uchiha displaying self-interested tendencies, sacrificing the many for the betterment of the few, specifically the clan or an Uchiha's close intimates. The events leading up to the Uchiha Clan Downfall can be considered the ultimate display of the clash between these two ideals: the Uchiha, feeling mistreated by the Senju-influenced government of Konohagakure, planned to perform a coup d'etat to take control of Konoha, paying little mind to the fact that this in the short term would start another ninja world war and would likely see the complete eradication of the Uchiha. Although the Third Hokage attempted to negotiate and spare the Uchiha, Danzō Shimura reasoned that peace was no longer an option and secretly arranged for the clan's massacre.
Although many Uchiha do display the Curse of Hatred at some point in their lives, some are able to transcend their clan's usual narrow-mindedness; Kagami Uchiha, Kagami's descendant Shisui, and Shisui's best friend Itachi all live by the Will of Fire instead. Obito Uchiha initially displayed characteristics of the Will of Fire, but he was corrupted by Madara Uchiha to live by the Curse of Hatred. Kakashi Hatake theorised that Obito never lost his Will of Fire despite falling to hatred, and the inner conflict between the ideals prevented Obito from fully committing to and acting on the Eye of the Moon Plan when he had the chance.
Individual Uchiha can also be released from the Curse of Hatred by those who are close to them, though this is incredibly difficult as the Uchiha becomes largely unreasonable and unflinching in their singular desire. After several years of failed negotiations, Hashirama Senju was able to draw his best friend, Madara Uchiha, away from the Curse of Hatred and channel his energies into their newly-formed Konoha. This was only a temporary success as Madara eventually returned to his former ways, but finally acknowledged that Hashirama was right before his death in the Fourth Shinobi World War.
Sasuke, the sole survivor of the clan's downfall, initially bore the entire clan's hatred upon himself in his desire to avenge them. Despite predictions that their respective embodiments of the Will of Fire and Curse of Hatred would cause them to destroy each other, Naruto Uzumaki was able to redeem his best friend. As Sasuke was, at that time, the only surviving Uchiha, the Curse of Hatred is effectively eradicated.