In Act 3, Scene 2, while waiting for Romeo to arrive on their wedding night, Juliet learns that Romeo has just killed her cousin Tybalt. The news puts her in a state of emotional crisis that changes her attitude concerning Romeo, but she resolves her emotional crisis by the end of the scene.
Juliet's first reaction to the news is to believe that she has been completely deceived by Romeo. She had based her love for him solely on physical attraction, believing that his beautiful exterior also contained an equally beautiful heart, mind, and nature. Now she feels the reality is that his beautiful exterior houses an evil nature, which we learn in her string of oxymora:
O serpent heart, hid with a flow'ring face!
Did ever dragon keep so fair a cave?
Beautiful Tyrant! fiend angelical!
O, that deceit should dwell
In such a gorgeous palace! (III.ii.76-88)
Since a serpent in terms of biblical mythology is known as an evil tempter, the phrase "serpent heart" refers to what Juliet now believes is Romeo's evil nature. Also, since flowers are beautiful, the phrase "flow'ring face" refers to his beauty, and collectively in this line, Juliet is accusing Romeo of having an evil nature that is hidden behind a beautiful face. Hence, all in all, we see that Juliet in these lines is expressing her fear that she has been deceived by Romeo.
However, when Nurse starts speaking ill of Romeo too, Juliet has a moment of revelation. She realizes that it's wrong of her to think poorly of her husband, simply because he is her husband. She then reasons that Romeo probably killed Tybalt out of self-defense. This moment of realization is also a moment when Juliet's love matures from mere infatuation to actual real love. Real love is a choice to love someone despite faults or other issues. Hence, Juliet's attitude toward Romeo changes in Act 3, Scene 2 to first an attitude of doubt, and then by the end of the scene she makes the conscious decision to continue to love him.