Believe it or not this movie, though not advertised as such, was created around a true story. For the true story go to movies based on true stories database
It was not advertised as a movie based on a true story or inspired by a true story. I just so happen to remember that case very well. It was a huge case. The Halloween murder, the mask, the photo of the mask, the killing of the pregnant woman, was what gave it away. The rest is fictionalized. I;m not sure why they do this sometimes. I guess the answer is that they either don't want to pay for the life rights to the story, or they don't want to follow the story that closely.