Twisted Vengeance by Jeff Bennington

A TWISTED KILLER IS UNLEASHED. 
While investigating a rash of unsolved murders, Rick Burns stumbles across a mysterious boy (if you want to call him that) who knows the killer. The ghostly kid reveals a dark underworld Rick never thought possible--a supernatural world where murder is extremely twisted.

The boy comes and goes, invading Rick's already complicated life, eventually leading him on a heart-pounding chase to the killer's lair. Rick doesn't know who to trust and eventually begins to question his own sanity. What Rick discovers is a maniac bent on vengeance that is unbelievably TWISTED. Twisted Vengeance is intense, frightening, and suspenseful. If you enjoy ghosts, poltergeists, mystery and the supernatural, this book is for YOU!. 

I dare you to guess who the killer is...  

A SERIAL KILLER DISGUISED.
Is the killer the unscrupulous wife? The Priest hiding in the shadows? A Poltergeist? The boy? An abusive father? Or is Rick something he never imagined he'd become, his own worst nightmare?

A HAUNTED PAST. 
On top of the already complicated case, Rick insists on keeping a promise he made to his dead brother. The promise? Prevent Stella from marrying another cop. The problem? Rick's in love with Stella and she's waiting for him to make his move. Unfortunately, Rick's mysterious past is haunting him, preventing him from dishonoring his hero. But could it be that his brother's death is related to the murders and that freakish boy? Are the answers hiding somewhere in Rick's childhood? He may never know until he releases his demons, and discovers who he truly is. And he'll never accomplish that or find the killer until he learns to trust Stella and the creepy kid that's tormenting his life.