Why did Kakashi kill Rin?

Rin Nohara and Kakashi Hatake are fictional characters in the popular manga and its anime adaptation, the Naruto series. At first, it was implied that Kakashi killed Rin by impaling her through her chest with his Lightning Blade. However, it was later revealed, in episode 371 of Naruto Shippuden, that when Kakashi was attacking Anbu from Hidden Mist Village, Rin intervened and came in Kakashi’s way and stabbed herself with Kakashi’s Lightning Blade.

In episode 371 of Naruto Shippuden, Kakashi and Obito are battling against each other. Kakashi gained an upper hand and had a chance to finish him off. Kakashi tries to persuade Obito to change his ways by trying to remind him of his old self. Kakashi compares Obito’s old self with Naruto. Seeing Kakashi’s hesitation, Obito taunts him and tells him the reason he can belittle Naruto’s ideals is that he used to be just like him and came to the realization that the world they live in is hell with no place for naive ideals.

Obito reveals that he knows the truth behind Rin’s death and then reveals that Rin was abducted by the Hidden Mist Village. Although Kakashi was successful in his attempt to rescue her, they were being chased by Hidden Mist Anbu. Rin then realizes that the Hidden Mist Shinobi sealed the Three-Tailed Beast inside of her and plans to unleash it when they reach Hidden Leaf Village.

They then realize the only reason they are being chased is to force them to get back to the Hidden Leaf Village. Realizing that this can cause the destruction of the Hidden Leaf, she asks Kakashi to kill her. Kakashi refuses her requests and tells her they will find a way to prevent that from happening.

Soon, they find themselves surrounded by Hidden Mist Anbu and to fulfill his promise to Obito, Kakashi confronts the Anbu unit to protect Rin. Rin finds the opportunity and comes in Kakashi’s way when he was attacking an Anbu Shinobi with his Lightning Blade. With Rin in the way, Kakashi ends up stabbing Rin in the chest.

Rin took this extreme step to protect her village and her friends from a potential Tailed-Beast attack, like the Nine-Tails attack which cost Minato and Kushina their life. Rin succumbs to her wounds. After this incident, people gave Kakashi the nickname, Friend Killer Kakashi. Everyone started to believe that Kakashi is a cold-hearted shinobi and would not hesitate to kill his friends and comrades for the sake of the mission. Although, in reality, Kakashi was willing to forsake his mission to save his friend and Rin sacrificed her life for the sake of the village.