Leveling Murder

Leveling Murder summary:

One day, Onitsuka Yamato, 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 resolves to survive - even if it means killing people, kill someone within a week or else he will die.