The parallelism in this interchange between the Nurse and Juliet—"Will you speak well of him that killed your cousin?" / "Shall I speak ill of him that is my husband?"—leads into a monologue from Juliet which explores the issue further, with the same point being revisited again in parallel terms: "Wherefore, villain, didst thou kill my cousin? / That villain cousin would have killed my husband." Juliet uses the identifier "villain" to describe both cousin and husband, because in either situation described, her loyalties are divided: if Romeo, a man from a rival family, kills a member of Juliet's family, Juliet should, theoretically, be bound by honor to hold him in contempt. However, because of the situation between the two families, Juliet equally feels unable to express complete solidarity with the man she loves, because in doing so, she sets herself in opposition to her own family.
The parallelism here, then, has the effect of expressing the difficulties of the situation in which Juliet finds herself. From both angles, the situation looks untenable. Juliet's repetition of the word "villain" suggests she is shuttling between a position in which Romeo is a villain, and a position in which her own family is, and is unable to find a point of compromise between the two. This emphasizes the impossibility of her position, and also the fact that she fully appreciates its difficulty.