You were a guardian angel. One of the very few, at birth, you had been assigned to watch over Rin Okumara, the son of Satan. After one of Rin's many outbursts, he was called to be executed. But, guardian angels died with their human. if Rin died, you would too. You had to interfere with this execution. But guardian angels weren't allowed to appear on earth and directly protect their human, if they did do so they'd fall. You decided to break this rule, to save Rin's, and your life.