The person to blame is Romeo. Because he first got Mercutio killed in the fight with Tybalt. He got in the way of the fight that distracted Mercutio and got him killed. Then, after Benvolio dragged Mercutio away, Romeo got in a fight with Tybalt and then Romeo killed Tybalt on the spot. After that, Romeo got banished by the prince, and was told if he came back, he would have his head.
After the banishment, The Nurse told Juliet that Romeo was killed, then told her that he was only exiled. Juliet was upset about the banishment, but was happy that he wasn’t killed by the prince instead.