Sasuke Uchiha awakened his Mangekyō Sharingan after a profound emotional experience triggered by the death of his older brother, Itachi Uchiha, and learning the truth behind Itachi's actions. Although Sasuke did not technically kill Itachi—Itachi died from exhaustion and illness after their battle—Sasuke's intense feelings of loss, grief, and the revelation that Itachi had sacrificed everything to protect him and the village were the key catalysts for awakening his Mangekyō Sharingan

. Specifically, after Sasuke defeated Itachi and Obito revealed the truth about Itachi's selfless motives, Sasuke was overwhelmed by sorrow and rage. This emotional turmoil and reflection on his brother's sacrifice triggered the manifestation of his own Mangekyō Sharingan, which has a distinct design different from Itachi's

. With his Mangekyō Sharingan, Sasuke gained powerful abilities such as Amaterasu (black flames) and flame control, as well as the eventual use of Susanoo, a giant chakra warrior. Later, Sasuke transplanted Itachi's eyes to obtain the Eternal Mangekyō Sharingan, which prevented blindness and enhanced his powers further

. In summary, Sasuke's Mangekyō Sharingan awakened due to the emotional trauma and revelation surrounding Itachi's death and sacrifice, rather than by killing a close friend or family member himself. It was the emotional impact of losing his brother and understanding Itachi's true intentions that unlocked this advanced Sharingan form