Exciting and believable. "That Darkness" would have made an excellent "Rizzoli & Isles" TV-movie (or real movie). Just change homicide detective Jack Renner to Jane Rizzoli and forensic investigator Maggie Gardiner to ME Maura Isles.

That Darkness (Gardiner and Renner, #1) by Lisa Black is the first book in a new series featuring forensic investigator Maggie Gardiner and Homicide Detective Jack Renner, who happens to be a vigilante killer on the side. (This is not a spoiler, it is in the book description.) Just like Ms. Black’s other series, this one is also set in my hometown of Cleveland, Ohio and it is very accurately portrayed.
Maggie is obsessed with every little detail of her job and has no life outside of work. When corpses start turning up with the same mode of death, she starts to suspect a serial killer. All of the bodies except one belong to life-long criminals which makes her fear a vigilante is operating in Cleveland and could possibly be a member of the police force.
Jack is on a mission to avenge a death in his past and as long he is searching for his prey, he feels the need to eliminate others who he knows from research will violently offend again. Jack knows Maggie is like a dog with a bone about the similar cases and hopes to find and complete his final mission before he has to leave town.
Once again Ms. Black gives her readers a book with very descriptive forensic details. The plot flows at a steady pace throughout the story. My only problem was that as interesting as I found Jack’s character and the debate about vigilantism, Maggie’s character was not very developed. I will be continuing with future books and hope that Maggie evolves more fully and I also want to know how Jack is handled.