Gungrave

This is the story of Brandon Heat, a man who descended into the dark underworld in the name of revenge. He was once a very silent, peaceful man, with a forbidden love in his heart. Then the underworld syndicate Millennion came, armed with a host of undead soldiers. Everyone began to die, including his friends and the father of his beloved Maria. He and Harry decide to join Millennion hopng to take it down from within. Then, sucked in by the lust for power, Harry turns to a betrayer

This is an awesome anime series so try to watch it!!!