What is an explanation of the following lines: "What devil art thou that dost torment me thus? / This torture should be roared in dismal hell. / Hath Romeo slain himself?"

When the nurse arrives with the rope ladder that was meant to permit Romeo to climb to Juliet's bedroom on wedding night, she must deliver the news that Romeo has slain Tybalt. However, she is not very good at delivering news: she describes someone's dead body, without saying whose exactly, and then she cries, "Romeo!" Thus, Juliet interprets this to mean that Romeo is dead, and she begins to panic. She calls the nurse a "devil" because she is tortured by the woman's inability to be specific, by the way the nurse fails to be clear about what has happened and who has been affected. She says that not knowing if Romeo is dead or alive is actually worse than being in hell. She vows that she will become deadlier than a snake if he is dead.

In this scene, the Nurse has just explained to Juliet that her new husband, Romeo, has killed her beloved cousin, Tybalt. Juliet, who knows Romeo is not prone to fighting, cannot believe that her true love would kill someone so dear to her. She turns on the Nurse, asking her what kind of devil she is to say such horrible things, and goes on to claim that the Nurse's news is more agonizing than the tortures of Hell.

Hath Romeo slain himself? Say thou but “ay,”
And that bare vowel I shall poison more
Than the death-darting eye of cockatrice.

Here, Juliet is saying that if the Nurse tells her Romeo has killed himself, Juliet will be so deep in her own grief as to become dangerous to others. She is not only afraid of losing Romeo, but also of losing her humanity.