The first series to be based on the exploits of a hired killer, predating both Loren Estleman's Peter Macklin and Lawrence Block's Keller, was the QUARRY series penned by Max Allan Collins. Quarry is a Vietnam vet who evidently learned his lessons all too well during his time overseas. Now he's home, earning a living as a cold, remorseless hitman, taking orders from a man called simply The Broker.