Books like Murphy's Revenge

Someone has started killing killers and Detective Martin Murphy has gone undercover in 'Confront', a support group for relatives of murder victims. He suspects them of carrying out revenge killings but it seems that they have been doing their own detective work and before Murphy realises, the group is forcing him to face the harrowing events of his own past. Now Murphy must come to terms with the past whilst also bringing the killers' killer to justice. The only problem is he's starting to think that whoever's doing the killing may have a point. After all, revenge is sweet... isn't it?