Why does Shakespeare kill Romeo and Juliet at the end of Romeo and Juliet?I'm doing an essay on this, and I have to have 3 main topics. All I have...

I certainly think that you are on the right track with why
the ending of the play breaks down the way it does.  In trying to show how tragic
conditions and proportions can be applicable to regular people, not only royalty,
Shakespeare has to end up killing both of the children to demonstrate such a truth. 
Within the Elizabethan time period, tragedy and the full dimensions of drama were only
seen in the realm of pure royalty.  While both children are a part of Veronian royalty,
their love compels them to act in a manner distinct from regal traditions.  In this
light, both of them can be seen as regular people and the idea of such individuals being
able to experience tragedy in their own setting might have compelled Shakespeare to have
them both die at the end of the drama.  Along these lines, such a reading helps to
remind us that true tragedy happens as a combination of circumstance and fortune, as
well as fatal decisions.  Neither character demonstrates a form of hamartia or some type
of tragic flaw.  In demonstration of this, both of them have to end up dying at the end
of the drama.