Leveling Murder

One day, Yamato Onitsuka, an office worker, kills another man who attacked him. When he took that man's life, a sound effect rang in his head informing him of his own strengthening and the acquisition of a spell. He had become a "murdelizer", a being whose level goes up when he kills someone. Yamato Onitsuka resolves to survive even if it means killing people as and to keep on living he needs to kill someone within a week or he will die