Do the characters have the ability to choose what they want to do or are they simply destined to participate in death and destruction?

I do not in any way believe that people have destinies
that cannot be avoided.  Of course, there is no way to prove this one way or the
other.

Now could Romeo and Juliet have chosen differently? 
Of course they could have (except for the fact that Shakespeare was controlling them and
they had to do what he wrote.).  I mean, nothing said that they had to keep trying to
see each other after they had found out they were from enemy families.  Nothing said
that they had to sneak off and get married.  They may talk about fate, but I think they
are just using that as an excuse to do what they want.

Of
course, the story would be way different if they hadn't and it would not be famous.  But
it wasn't destiny, in my opinion, it was their own
impulsiveness.