I would blame the Montagues and the Capulets both for more or less the same reason. Both families were too obsessed with their feud. The Capulets, specifically, are also to blame because they were forcing Juliet to marry Paris.
The Prince is at fault because he should not have banished Romeo. He should have understood the circumstances of Romeo killing Tybalt.
Mercutio is at fault for getting himself killed. Romeo did not want them to fight. Mercutio should have just backed off and not tried to fight Tybalt. Then Romeo would not have been banished.
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.
In the play "Romeo and Juliet" by William Shakespeare the author shows how several agencies (families,officials, the church) fail two young people and lead to a tragedy. The Prince "wink'd" at the feud for too long and did not do enough about it. The Capulets took no notice of the warnings and Lord Capulet even called for a sword so he could join in instead of cooling things. The Montagues were just as bad by joining in rabble-rousing in the square. Mercutio did not like a quiet life, and enjoyed stirring things up and then sitting back to watch the fun - which later turned to hate and killing. At the end all the parties involved must listen to a debrief speech and decide where their responsibility fell.
BY TELLING HIM HE WAS WRONG FOR WAT HE DID.AND THAT HE SHOULD JUILET LIVE HER LIFE THE WAY SHE WANTS TO.AND DEAL WITH HIS LIFE INTESTED OF MESSING UP SOMEONE ELSE'S LIFE.