Juliet, who hasn't heard about the whole murder/ revenge killing thing, is watching the clock for nightfall, when Romeo is supposed to sneak into her room.
When the Nurse enters, Juliet realizes right away that something has gone wrong.
First, Juliet thinks Romeo has been killed. Nope: her husband has just murdered her cousin.
Juliet's first reaction is to curse Romeo, and the Nurse joins in—but you know that isn't going to go over well, and it doesn't.
Juliet turns on the Nurse and tells her she can't criticize her husband. If he hadn't killed Tybalt, then Tybalt would have killed Romeo. Forced to choose between the cousin she has loved all her life and her new husband, she chooses Romeo.
Just as she's decided to forgive Romeo, she remembers that he's been banished and starts flipping out.
Her Nurse promises to find him so they can at last say goodbye.