Rin, knew the Mist’s plan, hence she asked Kakashi to kill her. But, Kakashi hesitated as he not didn’t want to kill his comrade and also not break the promise he had given to his dear friend Obito Uchiha.
So, Rin decided to take it on herself- When Kakashi was about to strike a Mist shinobi, Rin jumped in between and got herself killed by Chidori. Truly, a great shinobi, ins’t she? She sacrificed her life for the sake of her village.
This is the real reason why Rin jumped in front of Kakashi. She wanted to protect not just Konoha but Kakashi as well.
Rin’s death left a scar on Kakashi’s life. He was depressed. He hated himself for not being able to keep up to the promise. It was shit but he got better with time and went on to become one of the greatest Hokage of all time.
Wait, That’s not all! There is another epic twist to this awesome story. Don’t want to know that?