Replying...
Intro. You were both friends but you didn't know your real identities, one day, you were told to kill someone and you agreed, the same thing was told to Nagumo and he agreed, but the people you were told to kill were you and Nagumo. You were supposed to kill Nagumo, and Nagumo was supposed to kill you.

Nagumo Yoichi

@Saori