Juliet's first response when she learns that Romeo has killed Tybalt and is banished is to revile Romeo, referring to him as a "serpent heart, hid with a flow'ring face!" She also calls him many things that are opposite of what they seem, such as a "fiend angelical," which is an angelic devil, and a "wolvish-ravening lamb," which is a lamb, that like a wolf, eats other lambs. These contrasting images serve to show Juliet's inner turmoil. She thought that Romeo was so handsome and wonderful, but now that he has killed her cousin, she feels that she has been deceived. We see this further in the line, "O, that deceit should dwell / In such a gorgeous palace!"
However, when Nurse also begins to revile him, declaring that there is no "trust," and proclaiming, "Shame come to Romeo!," Juliet changes her tune. She calls Nurse a "beast" for "chiding" Romeo. Suddenly, Juliet responds to Romeo's committed murder and banishment by saying that Romeo was "not born to shame" and that she should not "speak ill of him that is my husband." She even points out that her cousin would have killed Romeo, calling her cousin a "villain."
She also responds to Romeo's banishment in the same way that Romeo responds to it. She believes that it is a punishment worse than death. She argues that Romeo's banishment feels like her "father, mother, Tybalt, Romeo," and herself have all been murdered.
Hence, Juliet's response to Romeo's murder of Tybalt is two-fold. First, she responds in shock and horror, feeling that she has been deceived by Romeo, then she remembers that he is her husband and decides that she should not think poorly of a man who is her husband. Once she decides not to think poorly of her husband, she begins to grieve over his banishment.