Ideas on horror story?

I'd make him an insane man who lives alone and maybe his only child just died. That set him off so he decides to start killing people and begins with the main character. The main character is just a case of wrong place, wrong time. The killer, however, is now completely crazy. He was unstable and a loner before but when his kid dies, he goes nuts. You're novel would then be about a serial killer who gets started for no apparent good reason. Lots of books are about serial killers with dozens of victims. Few books have been written about how these lunatics start out.