This is the only missable achievement in the game, you can get it after the second fight with Zangetsu in the Oriental Sorcery Lab, after defeating him you will get his katana Zangetsuto. Now it is time to fight Gebel in the Hall of Termination. The trick is to NOT cut the moon when it turns red but kill Gebel instead. This will trigger the bad ending. Also, remember that there is a game+ mode so if you miss this one like me you can return to the fight within 2 hours or so.