This speech is delivered by Juliet immediately after she discovers Romeo has slain Tybalt. Juliet speaks this speech while mostly in shock, and the language is jumping from image to image as she tries to understand her world which has been permanently changed. I'll look at some of the specific lines and then offer up a possible thesis statement that could describe this speech.
O serpent heart, hid with a flow'ring face! / Did ever dragon keep so fair a cave? Beautiful tyrant! fiend angelical! / Dove-feather'd raven! wolvish-ravening lamb! (III.ii.76-79)
First, Juliet describes Romeo as a serpent, hidden by a flower. This flower image goes back to the balcony scene, where Juliet continuously describes Romeo, and their love, with the imagery of roses and flowers. Now, however, she sees him as a serpent, but in the next line, she views Romeo as a dragon. Yet, this imagery changes even more. After being a dragon, Romeo is then a tyrant, an angel, a raven disguised as a dove, and then a lamb that appears to be a wolf.
What can a reader or listener make of this shifting imagery? It depicts how Juliet is unsure of everything. She is confused and grasping at a solid understanding of Romeo's actions. By the end of the speech, Juliet has gone through many images but settled on the fact that Romeo may be a monster dressed as a god (or whatever image you want to go with).
A thesis statement could be: Romeo's murdering of Tybalt causes Juliet to question all of her notions of Romeo, although, despite his actions, she still views him as beautiful.