The Demon and the Girl
An angel and a demon both fell in love with the same human girl. But everyone said that the girl's existence would bring disaster upon the entire world, and that she... should never have existed.
To save the world and uphold what she believed was the greater good, the angel resolved to kill the girl.
When the demon found out, rage consumed him, and he became a bloodthirsty monster, swearing to slaughter everyone in the world and make them pay for the girl's death.
The battle lasted two whole years. At last, the moment came: the demon finally killed the angel and turned his blade on the surviving humans...