Code Red (Rogue Warrior Thrillers Book 1)

Bob Singleton used to be a top CIA assassin. Now, scarred by his terrible past, he lives in a refrigerator box behind a dumpster in downtown Chicago. He just wants to be left alone. But his past is coming back to haunt him, in the shape of a nurse and a teenage boy who desperately need his help. They are being pursued by trained killers because they stumbled on a long-buried conspiracy, a secret that is tied to a failed mission in Bob's military past. Now they need Bob to protect them. The old Bob, the one who fearlessly put his enemies in the ground. –amazon.com.