I hate that question, 'How do you get your ideas?' But for once I can answer.
Some years ago I was watching the news. A U.S school was in a state of panic after multiple shootings by an adolescent. The account was truly shocking and the outcomes were followed closely by the media over several days. Sadly, there have been other such events since, in and outside of the U.S. I knew that much analysis would follow. As a psychologist, I had sometimes interviewed/assessed such youngsters. Those individuals, confidentiality prevents me writing about but I could use the experience to imagine new characters in that dangerous role.
I imagined a younger boy into a different geographical and social setting and imagined what experiences might lead to such an extreme act. I wrote a longish short story. It was long-listed in the (now defunct) FishKnife competition that year. Later it won a Bloomsbury review from topping the favourites on the writers’ YouwriteOn site.
The Bloomsbury editor said that I "was a writer of potential" (pun), that I had "an intriguing premise", that my first line provided "a gripping opening" that "plunges the reader straight into the novel's moral dilemma" and that s/he "was impressed by use of a first-person narrator." S/he went on, "The use of an unreliable narrator is tricky to pull off, and you handle it well – the character of Jake has stayed with me since I first read it." There were also suggestions for how I might improve it, associating it with 'Before I Go to Sleep' and 'Gone Girl'. I put my story to one side, because at that time I was wholly involved with rewriting my trilogy, A Relative Invasion.
Kindle has the first of my Crime Shorts, A Boy with Potential. Will it be a killer? Try a weekend read -A Boy with Potential
It's a 5k read. I do believe there is an appetite for stories of that length. Indeed, one reviewer (Morgen Bailey) has written: "This story has a feel of The Curious Incident of the Dog in the Night-time, although I much preferred this one, and it just goes to show how much can be done in around 5,000 words.