In Act 3, Juliet says, "O serpent heart, hid with a flow'ring face! Did ever a dragon keep so fair a cave? Beautiful tyrant! fiend angelical!" What is the meaning of this quote?
As was mentioned in the previous post, Juliet learns that her husband, Romeo, has murdered Tybalt and is in a state of disbelief. She is overcome with anguish and responds to the Nurse by saying,
"O serpent heart hid with a flowering face! Did ever dragon keep so fair a cave? Beautiful tyrant! Fiend angelical!" (Shakespeare, 3.2.74-76).
Juliet feels like Romeo has tricked her and is saying that Romeo is like a snake disguised as a flower. She proceeds to compare Romeo to a malevolent dragon and asks if a dragon has ever kept such a "fair" cave. Metaphorically speaking, the cave that Juliet is referring to is Romeo's attractive body. Juliet then uses two oxymorons to contrast Romeo's opposite character traits. Despite the fact that Romeo is beautiful, his murderous actions are similar to those of a tyrant. Romeo also has angelic features but acts like a fiend. Essentially, Juliet feels like she has been fooled by Romeo's attractive appearance and believes that he is a malevolent person.
In Act III Scene I, Romeo kills Tybalt--Juliet's cousin--since Tybalt killed Mercutio. We also know that Romeo & Juliet are now married (in a rather hasty and secret ceremony). When the nurse brings news that Romeo slew Tybalt, Juliet responds with the words you mentioned, and a few more.
In her mind, she is hoping she has not been deceived by this Romeo guy; in other words, he might have a "flow'ring face" (meaning "handsome"), but actually have a "serpent heart" in him that would be cruel enough to slay her cousin.
Most of this emotional rant deals with contradictions, or oxymorons, as she tries to understand why such a great guy might kill Tybalt. In other words, he might be "beautiful", but he could still be a "tyrant"; he might be "angelical", but still be a "fiend". Basically, her mind is trying to process whether he is good or evil.