The first book in the Armageddon Angels series by Eric Swett (innocenteric).

Long ago, an Angel surrendered his memories and his power when he left Heaven behind so that he could make a difference in the real world. A couple of thousand years walking among the teaming masses of humanity has left him disillusioned, but he refuses to give up. He spends his time with the forgotten souls of an urban slum, making a difference wherever he can. 

When Justin witnesses the brutal murder of a woman at the hands of a ruthless business tycoon, he must run before his eternal life is cut short. Justin must swallow his pride in order to save himself, and the one friend he has left. The journey back to the fold is filled with danger and discovery as he finds himself in a race toward the end of the world.