I disagree on the basis that it takes a certain kind of person to write a story about a sociopathic killer. Murdering senselessly is seldom high on the list of qualities others want to advertise themselves with, and so few sociopathic killers will likely go out to write about themselves and their hobbies, but it still takes a certain kind of author to write that work, and to direct its flow.
But many of what I wrote are based on fictional stuff I watch/read/play. How many here have made stories about high school in Japan even if they have never experienced it?
That is a setting, not a specific subject or the crux of the story. If the point of the story was just "high school life in Japan", then even so you can surmise that the author has an interest in Japanese high school life. I, for one, find it nothing interesting and would never write anything like it. A mere setting already gives away an interest, an adoration or fascination.
I like to write about a sociopathic killer, but that's just because I have read some awesome fiction involving such characters and I get inspired to write one. Does that mean I like sociopaths?
You are twisting an earlier argument, though I cannot tell if you intentionally do so. "I wrote a story about a Dwarf who saves his Kingdom" does not mean the author is a Dwarf from the medieval fantasy times who just so happened to saunter into our world to deliver literature.
A work of literature is the sum of your experiences and interests; combining "why did that particular author use this theme?", "in what way does the author present the theme?" and "what outlooks do the characters have?" all reveal answers about the author as a person.
No person with a deep-seated hatred for murderers will ever write a story that makes a reader sympathise with a murderer; someone who firmly believes there is no "evil" and even killers have a cause and are normal human beings may feel like writing a story about a murderer if only to show people they can be wrong with their preconceptions, or for other reasons.