A number of Uchiha with the Sharingan, a by-product of the Curse of Hatred.
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 be transformed into hatred in the blink of an eye, 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.[1]
The beginning of the Curse of Hatred.
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.[2] Indra was overwhelmed with anger for being passed over and, once manipulated by Black Zetsu, waged war against Asura to claim what he believed was his birthright.[3] 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. Where 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 in the long term would likely see the complete eradication of the Uchiha. Konoha, in order to minimise the bloodshed, ordered the assassination of most of the clan's members.
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.[4] 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 theorises 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.
The Curse of Hatred comes to an end.
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 before Madara dies during the Fourth Shinobi World War he acknowledges that Hashirama was right in the end. Naruto Uzumaki is also able to convince his best friend, Sasuke Uchiha, to turn away from the Curse of Hatred[5] despite predictions that they would destroy each other.[6] As Sasuke is, at that time, the only surviving Uchiha, the Curse of Hatred is effectively eradicated.