Tensei Shitara Warui Kuni no Musume Deshita

Tensei Shitara Warui Kuni no Musume Deshita manga summary: Main character's childhood friend vanishes comes back to say goodbye to main character. Childhood friend gets attacked by something, main character dies in crossfire. Main character is reincarnated as the living weapon of the demon lord, while his childhood friend is apparently now the hero.