For a 'first kill' short story, consider a story about a werewolf. A young man discovers he is a werewolf one full moon. His first kill is an accidental one. He was out in the woods, unable to control his new form. He attacks a deer. When he wakes up the next morning, he is horrified by what he has done. The rest of the story could be about him learning to control his werewolf side and coming to terms with his first kill.