Even the Wicked — Lawrence Block

Matthew Scudder knows that justice is an elusive commodity in the big city, where a harmless man can be shot dead in a public place criminals fly free through holes in a tattered legal system. But now a vigilante is roaming among the millions, executing those he fees deserve to die. He calls himself "The Will of the People"--an ingenious serial killer who announces his specific murderous intentions to the media before carrying through on his threats. A child molester, a Mafia don, a violent anti-abortionist -- even the protected and untouchable are being ruthlessly erased by New York's latest celebrity avenger.
Scudder knows that no one is innocent -- but who among us has the right to play God? It is a question that will haunt the licensed p.i. on his journey through the bleak city grays, as he searches for the sanity in urban madness. . .and for a frighteningly efficient killer who can do the impossible.