I wrote a book about a child killer once and part of the narrative is from his point of view. There was/is nothing wrong with that book. In fact, it's one of my most popular. But now that I am a father, I would not do that again.
I wrote my first real murder story as a journalist for the Daytona Beach News Journal in 1980. It was about a body found in the woods. Later, the murder was linked to a serial killer who was later caught and executed for his crimes.