Why do you think the play ends with the last line from Anne's diary? "In spite of everything, I still believe that people are really good at heart."

Few, if any, can answer this.  I think that the ending to
the diary is what helps make it such a powerful piece of literature, in general.
Bringing it out as the ending to the play helps to create the sense of drama that is
such a part of the Holocaust. While the ending speaks volumes about the Holocaust, it
carries a sense of the profound, in general.  I think that the ending reflects Anne's
own maturation and her own growth throughout the diary.  At a point where there is total
madness in the world, one can see how the protagonist seeks to believe in redemption and
in human goodness.  This reflects an aspect of adolescent growth where the abstract is
embraced and the complexity in the world is dissected through individual perception. 
The ending to the diary reflects the level of thoughtfulness and intricacy in thought
that Anne has absorbed throughout the narrative.  It is difficult to ascertain why the
ending is the way it is, other than to indicate that Anne's mind would have continued to
generate profound thoughts had the Nazis not captured her.  In this ending, I think that
the true sadness of the Holocaust is revealed, in that some of the most profound of
thinkers were extinguished with little regard.  The play's ending in this light helps to
reinforce this.