Pick a character who has not yet killed anyone in his or her own canon, but reasonably could, or someone who has killed, but whose first kill wasn't covered, and write a story about that person's first kill.
Cornuthaum did a couple of paragraphs for Nanoha, Fate, and arguably Chrono here, but not full stories. If he lets you, you could use them as inspiration, or maybe he'll decide to do them, himself. That series is not the only option, though.
Cornuthaum did a couple of paragraphs for Nanoha, Fate, and arguably Chrono here, but not full stories. If he lets you, you could use them as inspiration, or maybe he'll decide to do them, himself. That series is not the only option, though.