A bit more information about the serial killer, from the message board on the IMDb page:
"Anyways, we know a few things about the killer. He's said to be possessed, or suffers from multiple personalities. He hears the voice of a demon that commands him to kill. He is also trying to keep this "dark side" from his family, leading him to live a double life as a family man and serial killer."
That can be taken a lot of ways. It could just be backstory told in flashbacks, because the killer is supposed to have been dead for 15 years. But there's some speculation that he escaped his execution and has been hiding ever since. Or there's another possibility - the killer has been dead for 15 years and it's spirit is possessing this family man.
Any way you look at it, it's gonna be gruesome.
Sarcasm is an allergic reaction to stupid people.