Juliet means that she loves Romeo, but he is frustrating her.
Juliet loves Romeo with all of her heart, but you can love someone and still be frustrated with him. When Juliet finds out that Romeo killed Tybalt, she is very angry. This is why she uses these three beautiful figures of speech to describe him.
First of all, an oxymoron is a pair of words that are a contradiction. People often think of “jumbo shrimp” for example. That one is my favorite! “Jumbo” usually means large and “shrimp” is something small, but a jumbo shrimp is just the largest shrimp, but it’s an oxymoron.
When Juliet finds out that Romeo killed Tybalt, she is really in a bind. Tybalt is her cousin and she loves him, but she knows Tybalt’s temper. She also knows that Romeo is not the fighting type. She realizes Romeo is probably the innocent party here. However, she also has another problem. She is secretly married to him!
His actions affect Juliet, and there is nothing she can do here. She loves him. He is the "Beautiful tyrant!" because his course sets hers.
O serpent heart, hid with a flowering face!
Did ever dragon keep so fair a cave?
Beautiful tyrant! fiend angelical!
Dove-feather'd raven! wolvish-ravening lamb!
Despised substance of divinest show!
Just opposite to what thou justly seem'st,
A damned saint, an honourable villain! (Act 3, Scene 2)
When she says that Romeo is the opposite to what he seems, and calls him “serpent heart, hid with a flowering face,” this does not mean that she really thinks that he is evil. It just means that she appreciates the severity of the situation. When she calls him “Beautiful tyrant! fiend angelical!” it is because he has a hold over her. He is deciding the course of her future with his actions.
The prince actually outlawed all fighting, so by fighting and killing Tybalt, Romeo really messed up. This is why he is a "damned saint" and an "honourable villain." He did nothing wrong, and was actually trying to do the right thing, but is condemned for his actions.
Notice that when the Nurse tries to disparage Romeo, Juliet comes to his defense.
Blister'd be thy tongue
For such a wish! he was not born to shame:
Upon his brow shame is ashamed to sit;
For 'tis a throne where honour may be crown'd
Sole monarch of the universal earth.
O, what a beast was I to chide at him! (Act 3, Scene 2)
Try though she might to be angry at him, Juliet still loves Romeo. She can’t be mad at him for long. She will stand by him. She refuses to let anyone else insult him, and she will not scold him anymore. Juliet decides that there is nothing else they can do for Tybalt. Tybalt is dead. Romeo is alive, and she loves Romeo. It is to Romeo that her loyalties lie.
In a way, Juliet has no choice. She could marry Paris, but she would know in her heart that what she was doing was not honorable. So she decides to trust Romeo, and hope that he will find a way to come back to her. In the meantime, she also puts her faith in Friar Lawrence, and comes up with a truly remarkable but also truly awful plan to fake her death. With it, both their fates are sealed.