The Reckoning by Jane Casey

To the public, he's a hero: a killer who targets convicted paedophiles.
Two men are dead already - tortured to death.
Even the police don't regard the cases as a priority. Most feel that two dead paedophiles is a step in the right direction.
But to DC Maeve Kerrigan, no one should be allowed to take the law into their own hands. Young and inexperienced, Kerrigan wants to believe that murder is murder no matter what the sins of the victim. Only, as the killer's violence begins to escalate, she is forced to confront exactly how far she's prepared to go to ensure justice is served...