The Curse of Hatred (憎しみの呪い, Nikushimi no Noroi) was 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 no 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.
There have been cases where the Curse of Hatred can be channel into something positive, although it may manifest in an extreme manner. Madara for example, after founding Konoha acted what he believed benefitted the village, but took a more ruthless approach in contrast to Hashirama's more compassionate approach. Sasuke Uchiha, up until he found out the truth about Itachi, decided to go against everything he thought Itachi was, a ruthless killer who killed people for no reason by doing the opposite: he double-crossed Orochimaru after he outlived his usefulness, claiming disgust about how Orochimaru toys with people's lives for selfish and personal gains and avoids killing whenever possible.
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, Shisui's best friend Itachi, and Itachi's niece Sarada 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. In the anime, Fugaku Uchiha lived by the Will of Fire and tried everything to maintain peace in Konoha. Even when he agreed with his clan's desire for revolution, he expressed his dismay at going about it in a way that involved the bloodshed of civilians, hoping to end it quickly and with as little harm as possible. He also instructed his clansmen to leave Kakashi alone when they expressed their desire to forcefully take back his Sharingan, which Obito had left for Kakashi.
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 Uchiha, the sole survivor of the clan's downfall and Indra's latest reincarnation, initially bore the entire clan's hatred upon himself in his desire to avenge them and eventually became so submerged in its darkness that all he wanted was destruction. However, after meeting a reincarnated Itachi, Sasuke's resolve was shaken due to Itachi professing his undying love for his brother. This was enough to throw Sasuke into confusion and sought answers from the previous Hokage. As a result, Sasuke was partially brought back from the darkness after hearing Hashirama's story and Hiruzen's explanation of the Uchiha Clan Downfall. As a result, Sasuke's ideology become a mixture of the Curse of Hatred and the Will of Fire as he intended to execute the Five Kage in order to make the world focus its hatred on him instead of each other, thereby achieving a revolution in contrast to the pure destruction he previously craved. Despite this, Naruto Uzumaki, Asura's reincarnation, was able to redeem his best friend and purge the Curse of Hatred from him completely. As Sasuke was, at that time, the only surviving Uchiha and his daughter, Sarada, embraces the Will of Fire, the Curse of Hatred is effectively eradicated.