In Act 3, Scene 2, why does Juliet use so many oxymorons?
It is actually Shakespeare who uses so many oxymorons, such as the following:
Beautiful tyrant! fiend angelical!
Dove-feather'd raven! wolvish-ravening lamb!
A damned saint, an honorable villain!
Shakespeare has Juliet doing this for several reasons. One is that it illustrates her confusion about her feelings. She loves Romeo but blames him for killing her kinsman Tybalt. She has to incorporate both feelings into her young, sheltered view of the world and of humanity. The oxymorons are also a poetic novelty in this play. They afford some amusement to Shakespeare's audience while at the same time showing Juliet's reaction to the bad news. She is just beginning to sense the possible consequences of the fatal duel. It may mean that Romeo will be executed. It could have disastrous consequences for her marriage and her newfound happiness.
Romeo's duel with Tybalt marks the turning point in the play. Juliet's reaction shows that reality is already intruding into the illusion of perfect love. This young girl is torn between her ties to her family and her new ties to her handsome and passionate young husband. The oxymorons mainly serve to illustrate her confusion and mental anguish.