Blackjack. The Erin O'Reilly Mysteries, #29

Every action has consequencesWhen a young man is found outside a tattoo parlor with his skull crushed, not even the NYPD cares very much. The victim, a violent street thug and accused murderer, won't be missed. But after a second man is slain, Detective Erin O'Reilly becomes concerned. Dangerous men who have evaded the law are being killed by a shadowy avenger. Erin and her squad are faced with an unusual adversary; a killer determined to kill only bad guys.
But when a man appoints himself judge, jury, and executioner, he has to be stopped. Erin, her K-9 Rolf, and the Major Crimes unit buckle down to the unpleasant task of bringing in a suspect who genuinely believes he's doing the right thing. Law, justice, and revenge collide as Erin's team track their quarry from the streets of Hell's Kitchen to the marble hallways of New York's courts. Erin will have to confront not only a self-righteous murderer, but her own sense of right and wrong.  And meanwhile, in the background, an old enemy is maneuvering to bring his own brand of street justice crashing down on Erin herself.