The man raises Redbone coonhounds.  He lives in a backwoods mobile home, alone, save for the awful memories of Vietnam.  He cradled dying men, bagged bodies of fellow Marines, killed with vigor and was wounded three times.  Most of these horrors have faded to shadows.  One, his belief that he was responsible for a friend's death, seems indelible.   The elderly couple are in their 80s now, their children long grown and dispersed.   Their dead son, their second oldest, a victim of Vietnam, is never far from their thoughts and prayers.  They now know how their son died, that another Marine believes he is to blame.  Several years ago they met with this beaten-down survivor of war.  "We don't hold you responsible for our son's death," they emphasized to him.   Their words, while easing the veteran's anguish, have not erased it.  In recent years, the elderly man has become a surrogate father to the aching man.  "He reminds me a lot of my real father," says the former Marine.