Ha! I can see how this could be confusing, especially to a first-time reader. No, Juliet doesn't want to kill Romeo. (Although it is ironic that almost everything spoken of here does, in fact, come true.) Juliet is covering for herself here. She is hiding her love for Romeo by pretending to grieve for Tybalt. Some of her grief has to do with the loss of her cousin, yes, but most of it revolves around the fact that Romeo (her love) is the one who killed Tybalt (her cousin), not to mention the fact that this murder happened right after Romeo and Juliet were married and that this conversation happens right after Romeo and Juliet consummated their marriage. (Yes, Juliet is truly messed up.) Juliet gives us a warning through the "aside" only a few lines above the ones you mention:
Villain and he be many miles asunder.--/ God pardon him! I do, with all my heart; / And yet no man like he doth grieve my heart. (3.5.86-88)
Here, Juliet is letting the reader know that she hopes that God forgives Romeo as she already has. The knowledge of all of this brings her great sadness. However, my favorite line is the following:
Indeed I never shall be satisfied / With Romeo till I behold him--dead--/Is my poor heart so for a kinsman vexed. (3.5.98-100)
This is a marvelous play on words on the part of Shakespeare as he has Juliet continue her ruse in front of her mother. Juliet wants her mother to hear that Juliet won't be satisfied until Romeo is dead. However, Juliet herself is using the opposite meaning: that she will never be satisfied until she simply "beholds him" and holds him close. It is her "poor heart" that is dead in this case, because she is "so for a kinsman vexed." Gosh, I love Shakespeare.
I couldn't end this, though, without sharing one final idea that is so important to the congruence of this scene. Romeo does, in fact, die of a similar poison of which Lady Capulet speaks. Juliet does, in fact, behold Romeo dead and, thus, takes her own life. Shakespeare was a genius in his connections, wasn't he?
To "die" during the era meant to have an orgasm. Juliet was using a play on words.