Here's why:
* Copyright Protection: "And Then There Were None" is still under copyright. Even though Agatha Christie died in 1976, copyright protection for her works extends for a significant period after her death. In the United States, copyright lasts for the life of the author plus 70 years, so her works remain protected until 2046.
* Derivative Works: Creating a modified version of a copyrighted work, even for non-profit purposes, constitutes creating a derivative work. Copyright law grants the copyright holder exclusive rights to create derivative works.
* Fair Use: The "fair use" doctrine allows for limited use of copyrighted material for purposes like criticism, commentary, or parody. Your fanfiction wouldn't likely qualify for fair use since it's primarily based on and derived from the original work.
What You Can Do
* Write Original Fanfiction: The best course of action is to create original fanfiction based on your own ideas and characters.
* Seek Permission: You could attempt to contact the copyright holders (the estate of Agatha Christie) and seek permission to use the work for your fanfiction. However, obtaining permission is unlikely given their strict copyright protection policies.
* Focus on Public Domain Works: Explore works in the public domain that have no copyright restrictions. Many older works are free to use and adapt.
Remember: Using copyrighted material without permission is illegal and can lead to legal consequences. It's crucial to respect copyright law and find legal ways to express your creativity.