There are a few reasons why Desiree might have made the choice she did at the end of this story.  First of all, when Armand turned against her, she was in complete despair.  He new famiy had become her whole life, and she couldn't bare the thought of living without Armand.  Secondly, Armand tells her that she is not white.  Even though she questions this assertion, she has no proof because she was an orphan.  In the time period of this story, to be part black would put you at the same level as the slaves, so Desiree would be scorned wherever she went from then on.