Cross the line - James Patterson

Shots ring out in the early morning hours in the suburbs of Washington, DC. When the smoke clears, a high-ranking cop lies dead. Under pressure from the mayor, Detective Alex Cross steps into the leadership vacuum to investigate the audacious killing. But before Cross can make any headway, a wave of murders erupts across the city. The victims have one thing in common - they are all criminals. And the only thing more dangerous than a murderer without a conscience is a killer who thinks he has justice on his side.