If you HAD to, how would you blame the Prince, Montagues, Capulets and Mercutio for the outcome of Romeo and Juliet play.this is if you absolutely...

Personally, I'd add Romeo and Juliet to that list, too,
but here is why I think the ones you've mentioned should be blamed for the outcomes in
William Shakespeare's tragedy, Romeo and Juliet.  First, the Prince
should be blamed because he did not need to banish Romeo.  Also, if he had just put an
end to all of the violence and ridiculousness of the feud before Tybalt's death, none of
the subsequent events would have even happened.  Mercutio was being hot-headed the night
he fought Tybalt.  He should have known better than to fight him, especially when he
must have known how much it would bother Romeo.  He was not being very loyal to him at
that moment.  Obviously, most of the blame should be placed on the Montagues and
Capulets.  Their feud had been going on for so long that its main cause had been
forgotten.  They simply knew that they hated each other, and that hate kept manifesting
itself from generation to generation.  If they would have just gotten over themselves a
bit and been mature, their children would not have died.