Why was the woman described as "nightmare" and "life-in-death" in "The Rime of the Ancient Mariner"?

 "The Rime of the Ancient Mariner" is the story of the
eventual redemption of the mariner for his senseless act of killing an albatross while
at sea.  Almost immediately after killing the bird, the wind stops and the sun beats
down on the other men on the ship.  They very quickly become dehydrated -- ironically
surrounded by water they cannot drink!  In Part 3 of the poem, the situation is becoming
more dire.  The descriptions of the men with their "throats unslacked, with black lips
baked" is a vivid image of how close the men are to death.  It is at this point that the
mariner sees what appears to be two figures:  Death and Life-in-Death.  This ominous
event suggests the eminent death of the men.  The glosses at the side of the poem, also
written by Coleridge, tell us the the two characters havebeen playing a dice game and
the winner of the game "wins" the mariner, while the other gets all of the other men. 
In this case Life-in-Death says, "The game is done!  I've won! I"ve won!" and she is the
winner of the mariner meaning that the mariner will now have to suffer through his life
with the memory of his actions and results of those actions for the rest of his natural
life.  Death loses, and therefore, he gets all of the other sailors.  In the conclusion
of this section, all of the souls of the dead sailors fly past the mariner as they leave
their human corpses.  It is very strong image that the mariner will haveto live with
forever.  This life that he still has is a kind of life in death because he will never
be able to leave this whole experience behind him.  He calls the figure a "nightmare"
because she represents the nightmare that his life will be.  Remember that the mariner
is doomed to be compelled to retell his story to strangers so as to relieve his
sometimes overwhelming guilt.  He is never allowed to just put it all behind
him.