The Blue Nowhere by Jeffery Deaver

His code name is Phate—a sadistic computer hacker who infiltrates people's computers, invades their lives, and with chilling precision lures them to their deaths. To stop him, the authorities free imprisoned former hacker Wyatt Gillette to aid the investigation. Teamed with old-school homicide detective Frank Bishop, Gillette must combine their disparate talents to catch a brilliant and merciless killer.