BRZRKR by Keanu Reeves

A WAR WITH NO END, A MAN WITH NO LIMITS.
The man known only as B. is half-mortal and half-God, cursed and compelled to violence...even at the sacrifice of his sanity. But after centuries, B. may have finally found a refuge - working for the U.S. government to fight the battles too violent and too dangerous for anyone else.
In exchange, B. will be granted the one thing he desires: the truth about his endless blood-soaked existence...and how to end it.